To all those who bought a 6D and enjoy it, congratulations, I do as well!
Apologies to Dustin, still have not read your review, but plan to. No doubt you could have reached similar conclusions.
Now that I have around 3500 shutter actuations on my 6D, I will just state simply, the reasons I chose a 6D over 5D3:
2) I decided even if the resale value of the 6D declined more of a percentage than the 5D3, relative to their lowest street prices for new units (which may or may not ever be the case, time will tell)...that it would still be almost inconsequential, given the extra $1k or so needed to buy the 5D3. If there is a point in the future where you would lose more dollars in buying a 6D and holding/using it for say 2 or 3 years...than the dollars you would lose over that time if buying a 5D3...I can't see it. The difference isn't all that much, but the advantage may be with the 6D here. From a dollars and cents standpoint, I suppose the more difficult choice, would be between buying a used 5D3, or a new 6D...if you are inclined to buy a used body. Refurb camera body prices are usually a bit too close to the new price, for my taste. But not always.
3) The reviews looked like the image quality between the two, was very similar at all ISO's.
4) So I tried my cousin's 5D3, and then edited several of his RAW files, both on his computer, and on mine. He even produced a large print of one of his landscapes, that I edited on my computer, shot at low ISO. However, I found that the 5D3 has very strong, very large grain luminance noise which shows prominently by ISO 4000. I found an adjustment of the luminance NR slider in LR4, needed to be high up at 80, to have any effect on this noise. And when it did, of course huge amounts of detail were erased along with it. So, despite what several reviewers and members on here may have said...at higher ISO, I'm sorry but there IS NO resolution advantage of the 5D3, over the 6D. There might even be a tie...or the 6D might even get the nod...in specific situations. Certainly I will admit that below ISO 2500 or so, the 5D3's resolution advantage becomes easily noticeable. But that resolution, for me is certainly not worth an extra $1k.
By contrast, my 6D doesn't have luminance noise that strong with its grain that large, until ISO 16,000. 4000 vs. 16,000. That's quite a difference! They both have similar amounts of chrominance noise throughout the range, and it is easily managed with noise reduction, until ISO 12,800...where it starts to get more blotchy on both cameras. Basically, when you get in this amount of gain, you learn to do without the nicest color rendition.
5) The 4.5 fps vs 6 fps is not enough of a difference for me, even when shooting sporting or other action situations. The real difference is when you go above 8 fps, in such situations (having rented a 1D4). The 5D3's higher buffer storage would be an advantage, but again…not worth the price difference for most of my usage. Certainly the extra card slot is a 5D3 advantage, as well as its usage of CF cards in the main slot.
6) Yes, the AF of the 5D3, is definitely worth the price difference, but only IF you feel you really need it. I do not. Just some minor tweaks of the 6D's servo AF via the menu, have made the camera responsive enough...that only a handful of my multi-shots have ever come up slightly out of focus. I've even tracked bats in flight at dusk (obviously more than a handful of those wind up a tad soft!). If sports photography is where I made my living...then of course I would buy a 1DX, and have a 5D3 or two as my backup. It's not my primary work. A 5D3, also is definitely not a precursor for birding. The 6D has done a fine job of tracking birds in flight with a 70-200 f/4, as well as my other "non-super-teles". As for tracking things as slow as people walking...it just feels like the 6D is not even under a strain....like it's idling...even in low light in servo mode with either an f/4, or faster lens. Granted, if the people are wearing dark, low contrast clothes, and the lighting is dark...and you don't put an AF point on their face...then that might be asking for trouble. But it would be the same for the 5D3. Its wider AF array with more points, would give it an advantage, but in very low light, that advantage is probably gone.
I guess my main conclusion, is that I can't understand why a wedding photographer (I am not one, but I know there are tons of you on here), would feel the need to use a 5D3 for shooting still shots, over the 6D. Even tracking a bridal bouquet in flight, would be child's play for the 6D's autofocus. If you feel you need the slight pixel resolution advantage at lower ISO, I guess I can understand. If the 5D3 makes better use of a flashgun's focus assist beam in such a situation...in servo AF...than does the 6D...I guess I could also certainly understand that. I have no idea if this is the case, though. Most of what I have seen at weddings, they're using off camera flash with box or diffusor...so not sure if AF assist beam firing even works in that situation (I admit it seems like it might be possible, I just don't know).
So, the only real and practical advantage I see with a 5D3, over the 6D, in a wedding situation…would be the ability to shoot superior video, without the moiré. But for still shots, I fail to see how you’re going to make full use of the 5D3’s AF system, at a wedding. Unless, during a wide angle shot, there is some kind of elaborate group dancing situation, where people are running across in front of the camera from left to right at unpredictable times, while at the same time moving fore and aft (to use the wider AF point array, with all its points activated, and try to lock onto the unpredictable action as it swishes about.) But then, in that situation…you might have the aperture closed enough that it wouldn't matter...the zone technique. Or even more likely, you might just be shooting video with the 5D3, instead of stills. Still shots don’t usually convey dancing as well as video...with sound...