I had 5D Mark II, sold, bought 5D Mark III, sold, bought 6D. Reasons are not important, but here are conclusions:
- 6D is quite a bit better than 5D Mark II in low light. Everyone says it's a touch better than 5D Mark III, I haven't confirmed that yet, but it's no worse.
- AF tracking is somewhere between mk II and mk III. I don't do sports, or wildlife, but chase toddlers, and my percentage of success is better than mk II, and maybe a bit worse than mk III. Kids seem to be so unpredictable at close focusing distance that I couldn't wring out much better results with mk III. Having said that, I shoot ONLY wide open, and with ridiculous depth of field (1.4 etc...), so I'm making it hard on camera and myself.
- AF accuracy is better than mk II. I _think_ it's not as good as mk III on outer points, but on central point, it's exactly the same thing, if not better. I shot mostly inside, so this tilts playing field in 6D favor, as you're probably aware.
- I don't mind 4.5(6D) frames vs 6(mk III), but again I'm not into sports. Something I'm missing is that mk III has hi-speed mod and 3frames/s mode, and 6D has only one. At 4.5 frames it's a touch hard to fire only one shot, often I fire 2 when I didn't want to. Quiet mode drops it to 3 frames in both cameras, but shutter lag is longer, which I avoid with active toddlers.
- Sync speed 1/180 vs 1/200 is red hearing. 1/180 is either enough or it's not, and if itsn't, you go into HSS mode in both cameras.
- I actually prefer 6D handling. It's very solid, nice and NOT small, but lighter than mk III. I didn't mind mk III, but when lugging some of the heavier lenses around house for awhile, I does get a bit hard on the hand, and I found I appreciate those 200g less. It's nothing decisive, just a bit of a comfort.
- I was always writing all files on both cards in mk III, can't do that in 6D, but since I'm not making money with my camera, I can live with it.
- I LOVE EOS remote app. I attach pic of my 18-moth old triggering the shutter while I'm holding camera to take a picture. Not sure it will be truly usefull, but it's cool