In all seriousness though, test drive both the 6D and 5D3 and figure out which one actually fits your needs better as they both have different pros and cons. Either one will be a significant upgrade from your current body and you may find that the 6D is enough to give you what you need + some other features that are not in the 5D.
Re contemplating on waiting for the 5D4.... your time could be much better spent on other things.
I have a 6D, which I'm very happy with. There are a few features I would like from the 5D3, but I wouldn't want the extra weight. That is personal preference, so you may reach a different conclusion.
Also, the price difference between the two would be half way to paying for a 70D, which you could use for video. Of course, that only works if you are content with full frame for stills only, and a crop sensor for video. Alternatively, any money not spent on a depreciating body can always be spent on lenses ;-)
Don't forget, you're not enjoying your investment until you go ahead and buy it. Buying a 5D3 or 6D now will save money compared to their street prices after launch, and even more compared to a 5D4 when that arrives. That may not be soon, if nothing comes out at the Autumn trade shows, then there is no point in waiting.
I agree with both of you here. Of course if you rent a 5d3 and a 6d, which is the more robust camera with more features...the 5d3. The question I am wondering though, which all the purists here don't seem to want to ask or care to know --- what is the OP shooting. He said 50/50 stills and video. but that's pretty much all. what kind of stills work? And what kind of video work? Demanding professional work? Semi pro? Or hobby?
that's why i bring up the 6d as an option - the 6d IQ is a huge upgrade over the 600d. Does it have the super AF system, yes. Which btw, the 5d3 AF is awesome, but, if your shooting landscapes on a tripod and using live view for focus then why spend the extra $1000 on the mk3? And with the the servo mode, unless your regularly tracking fast moving subjects - like shooting sports or wildlife, having the AF in the bag is nice but is it worth spending the extra $1000 for it? I shoot weddings and portraits, it's rare that i use the tracking capabilities (I don't need 100 shots of the b&g coming down the isle, using servo mode on that would pretty much be spray and pray). I also shoot events and live music, and every once in a while you get a metal band and they like to run around the stage - Servo does come in handy then, but even with that said, for the 2 or 3 shows a year this happens, not worth it. what sells me on the mk3 is that it is more rugged, better weathersealed, and does have some bells and whistles I need.
With that said though, when I got the 6d, I used it mainly with the wide angle lens, it was the machete, while the mk3 was the scalpel. Now after a few months using both, I care less about which body I use and more about the lens on each. I am now favoring the 70-200 on the 6d, and the 24mm 1.4 on the mk3. I also prefer going macro with the 6d because I don't have a grip on the 6d so i can get lower with it. where my weddings where once a 70/30 split (70% 5d3, 30% 6d), now it's more like 50/50 between the 2, and yeah, sometimes the 6d ends up pulling the bulk of the load.
the little things:
Sync speed has not been an issue. the 1/4000 limit, every once in a while this is an issue shooting midday if I want to be wide open, but, not so much of an issue that I hate it (ok, if i did not have a mk3 in the bag, this would be a bigger issue). Lack of a sync port, I thought this would bug me, but now I rather like the idea of having both bodies set differently for receptions - the mk3 can go with off cam flash, the 6d can go on cam flash - this gives me a greater variety of unique looks.
I do like the weight and the size of the 6d too, it does really help ease the load on a 12 hour day.
None of what is said here is in any way meant to bash the 5d3, it's a fantastic camera. Hell, one of the reasons i am using my 6d more now is to put less miles on the 5d3. What i am saying though is this - do you need everything the 5d3 brings to the table? If you were upgrading from a 7d, and use the 7d's AF system to the maximum level, if you shoot sports and wildlife, then yes - the 5d3 is probably the better choice. But if your not doing that, then, snag a 6d and use the savings to buy a nice lens!