Well I shot Nikon for 25 years, and changed to FF Canon in 2005.
The change to Canon wasn't just driven by FF. Some of the things I found with the modern Nikons would be my reasons for choosing the 6D over the D600
These only relate to the prosumer grade of cameras:
Software that's a joke compared with Canon
Hideous plastic pop up flash
Rubber grip that peels away from the body
Rubber grip that binds the front command wheel
Soft lens mount that wears quickly and feels real slack with heavy lenses
Translucent LCD on focus screen makes manual focus horrid
Menus that I don't understand
And, the manual focus on Canon L lenses is superior to Nikon IME
In fact I have found that the Canon has a higher level of intrinsic quality to the equivalent Nikon.
Nikons are cheaper because they Are cheaper.
Don't take any notice of people who have called the 6D "cheap and plasticy". They will have never handled one.
Well said Sporgon. I didn't shoot Nikon for 25 years, but I shot a Nikon system long enough in the film era to be a dedicated fan at one point in time. About 6 years to be exact, something I can't deny or regret. It certainly helped me jumpstart my dedication to events coverage. However, I was attracted to Canon when I began to see all the white lenses in highly inspiring and motivational events like the Olympics, presidential campaigns, and at local sports games in my hometown of San Francisco. It certainly made me very curious. Deep down inside, I wanted the speed, accuracy, and color renditions of the rapidly improving EF lens lineup. However, what really broke me in half with Nikon was when Canon released the ever so popular 5D classic. Everyone who's shot DSLRs for at least close to a decade know how much of an impact this particular model made when it was announced, and even bigger when all of the jaw dropping photos turned up. I can't really say that I switched, but when I bought my 5D with a 50mm f/1.4 my Nikon F4 never saw the light of day again after that. I had to sell it and settle for a 40D at one point but I bought the classic again when the Mark II got released. I kept loyal to the 5D series, and backed it up with a 1D mark III for shooting weddings eventually. When the 5D mark III came out I just had to do a double take and convince my boss to buy another one because the AF is just so buttery smooth with Canon lenses when shooting events. Then the 6D came out... I have to admit, I was disappointed. Canon gives us this war beast called the 5D mark III and puts a little innocent kitty cat next to it called the 6D. So I thought... Okay.... maybe my wife will like it. Maybe we can take it on vacations because it's smaller and lighter. Okay...it's so hard to defend against the D600 specs but we already have a lot of Canon gear...
After reading around in the Nikon forums, then going to the store to hold a D600 and try it out, I was disappointed in how it felt in my hands. Like Sporgon, the menus drove me nuts. I tested it in the store with a Nikkor 50mm 1.8 (which to be fair, feels better than Canon's 50mm 1.8 II BTW) Looking at the horrible LCD screen made me laugh, because it made realize that I was a film photographer that has become a chimper. But if I'm going to chimp, the LCD screen better be good. The slight green tint didn't bother me as much as I thought it would when reading about it. However, the image was cropped inside of the screen to make room for the settings... Wow. Even if that could be changed, it's really not a good idea to put that on a camera as default. Anyways, at that point, it really didn't matter to me what the Nikon images would look like on a computer screen anymore. I was holding a camera that an event shooter would have a bit more trouble using compared to a Canon system and even compared to the way my old F4 was. I'm used to my brother's D7000 layout, which I hate, and wish they changed it in future bodies. But nope, they brought almost all of it back. Also after that experience I have enough reason to believe that the live view/video mode is horrible on Nikon cameras in general. I'm also a cinematographer, and I am still facepalmed for Nikon for simple tasks like changing settings while in live view. I suppose they think everyone owns an entire set of AF-D lenses. This is an important feature with the most basic of functions that should have worked straight out of the box, seamlessly. The frustrating button placements and situations that caused me to menu dive constantly were enough to turn me off on just the camera alone. They should have kept it simple! These are vital operation constraints. Good photographers should be spending less time on the science of fiddling with controls, and more time on the art.
So I pre-ordered a 6D for my wife. I wasn't excited, of course. Being used to the 5D mark III, it was really hard to see that anything could come close. Now we have two Canon 6D bodies.... One for me, because my wife wouldn't let go of hers for me to really shoot all day with it. But the reason I have one now... The first impression. Holding it is NOT like holding a 60D. (which I've also owned more than one body of, previously) I think that's the common misconception by the general public who look at the specs and photos of the camera and say oh, it's a full frame 60D. It is not. It's definitely heavier, feels like I have a better grip on it, and the control layout seems to make slightly better sense. You will definitely notice that it is better built than a 60D from when you first pick it up. I have exactly one issue with it that the 60D might have a tiny advantage in. And that's the fact I can't program the SET button to go directly to white balance settings. I have a dedicated white balance button in the 5D mark III, why not the 6D? However, I said tiny because you can program the SET button for quick menu, and throw white balance into the quick menu. One whole button press behind, not that big of a deal unless you're shooting in a moving vehicle that passes different light temperature conditions in which you are required to shoot in at 5mph or faster. Seriously.
The AF is not comparable to the 5D mark III at all, and everyone is right in the regards of believing the very obvious truth that it is in a different league in that regard. But that's why people are going to buy the 6D, because not everyone needs the 5D mark III. Nor do they all shoot in death defying extreme weather. Back to AF, the 6D is no slouch. I can easily say it's AF is better than both the 5D mark II and 5D classic. In fact, I feel like it focuses faster than the D600 when I have a good lens attached to it like the 135mm F2/L or my colleague's 70-200mm f/2.8L II. Which is not a fair comparison of course, since I only used the 50mm 1.8 on the D600. I won't get in too deep with lens performance, but it does sort of make me feel like Nikon marketing is just throwing more points on the D600 to compensate for some of their lower performing Nikon lenses compared to Canon lens equivalents. Which of course, isn't true and I'm just a Nikon basher right? ~_^ Okay, so Nikon seems to have the edge on outer points for moving subjects. But in my 3-4 weeks of experience with the 6D, I can at least say that the outer points are very much usable in good light and a target with enough contrast, but it disappointed me for moving subjects. Note, this is the same disappointment I had for the outer points on the 5D mark II and 5D classic. Also note, I didn't know I had this disappointment until I became dependent on 1D mark III and 5D mark III outer point AF. So, if you're used to the old 5D bodies, or even the old 9 point system on the XXD bodies, you won't really feel a difference. The center point, however, is a miracle worker. With just a macbook pro lighting the master bedroom and not even facing the subject, I can lock focus and photograph my sleeping wife despite the fact I was forced to use ISO51200 to expose correctly at a hand holdable shutter speed. I'll have to test the same situation with a 5D mark III, but I know for a fact that even my 1D mark III couldn't do that or would hunt for a bit before it thought it could. The fact I'm able to lock focus on any shot that requires ISO51200 when my 1D mark III couldn't do it at it's highest ISO of ISO6400 changes the game a bit in that regard. I'm shooting a local wedding soon, and I'm confident enough to try the 6D as a backup body to the 5D mark III and see how it does in a very poorly lit church.
For high ISO performance, and sometimes, the way the images just looked at all ISOs... I was disappointed too, initially. I looked at some of my old 5D mark II RAW files, and they were obviously sharper when viewing the image full on. However, the softer images didn't bother me anymore after I saw how well it handled sharpening in Lightroom and unsharp mask in Photoshop. I loaded my 5D classic RAW files, and a lot of the ISO100 files looked cleaner somehow which I'm sure is because of the weak AA filter and bigger pixels. A friend of mine sent me a couple of D600 RAW files from one of his events, and they looked just as flat as my 6D RAW files. A lot of this is accurate towards what the general consensus is saying around the net with their tests. However, I don't care about the 100% crop tests because I don't deliver 100% crops to my clients. After editing all the files, I found that the 6D RAW files looked better than both 5Ds and still had more realism to human skin tone compared to the D600 RAWs. I can still praise the D600 in this regard, because despite the slight preference in skin tone, the D600 RAW files are quite good. However, I'm still in agreement with Sporgon on the fact I am not willing to trade Canon's weaknesses for Nikon's weaknesses. It's like trying to drive a ferrari with helicopter controls, it doesn't matter how fast your car is if you can't get it from point A to point B. Perhaps overboard with the metaphor, but you know what I mean. Canon warranty is better as well, but I won't get in too deep with that either. (Seems like I got pretty deep with everything else though, lol)
The bottom line is, you have to make sure you're buying the cameras from a photographer's perspective if you want it to do everything you need it to do. Like many say, the camera's eco system including lens selection and accessories is much more important, so narrowing this all down to just cameras will prevent anyone from making a truly open minded decision. Like I said earlier, I'd rather spend more time on the art of photography. I'm not a camera engineer, that's the camera maker's job. Specs, operation, and sometimes rumors, though they are important factors, are just a supplement to what it all comes down to and that's a good photograph. We're not supposed to be on the computer all day zooming in and shaking fists about dynamic range, we're supposed to be out taking photos. That's why I can easily recommend the 6D over the D600, because I know for a fact it does the job and doesn't give me long term issues that could slow down or harm my workflow in the field. But in conclusion.... All I can say is..... Just buy a 5D Mark III if you can afford it