Had the mkiii for about a year and loved it, but recently hit a bit of financial trouble and unfortunately with that value lying around it had to go. I missed it from the moment it went. Shortly after I purchased a mkii with some of the money. Mint condition and good value. However having now tested the two I am tempted to try and raise funds in the future and go back to the mkiii. It's in a different league.
To be honest a few of us here were puzzled why you took the risk of downgrading. The 5D3 is a proper upgrade to the 5D2 in every way and not just a sideway option as in a 6D. The 5D4 is probably 18 months away if you want to save up for that instead.
I'd heard a little about the mk4, I'll have to look into it. Yeah unfortunately there were bills that couldn't be met so there was no choice. Very reluctant sale and once im back on my feet I wouldn't hesitate for the mkiii again.
I wouldn't have said the 6D is a competitor to the 5D3 as they're totally different cameras (with almost £1000 between them). It's probably fair to say it's a gentle upgrade from a 5DII though with everything slightly improved (better low light AF performance, lower noise, better video etc.) but nothing groundbreaking. The 5D3 on the other hand will appear leagues ahead, especially with AF speed & accuracy but depending on what you're shooting (nothing I shoot moves) the 6D should suffice (although if the OP's struggling financially I'd personally wait for the next generation of cameras as the 6D and 5D3 have been out a while now either to grab a bargain as old cameras are sold off cheap or to simply get the benefits of what the next gen model have to offer).
6d is the natural progression from the 5d2, the mark 3 still appears to me to be closer to the 1 series since it's got great noise control, superb fast focus arrangements but is full frame. If you buy canny you'll find the differential between mk2 and 6d is pretty small price wise.
If you are into shooting video the 6D has issues with aliasing and moire even against the venerable 5D2, probably worse line skipping involved. The 6D has improved in certain area as Red has said especially in low light but it takes some hit such as max shutter speed at 1/4000s compare to 1/8000s in the 5D2 and a smaller viewfinder coverage of 97% compare to 98% . If these 3 features weren't compromised I would have rated the 6D as an upgrade and as in any upgrades there should not be any feature taking a hit. Clearly Canon had placed these restriction to protect the 5D3. It depend on your use but if you been using the 5D3 it is still a different league from the 6D and the 5D2.
Faced with the dilemma of wanting to upgrade to a MKIII but unable to justify the difference between the cost of the MKIII and what I'd been offered for my MKII, (and nor did I consider the 6D a worthwhile upgrade on the 5D MKII, whatever anyone thinks), I sold my Canon gear and bought a Nikon D610 with similar lens line-up. No regrets whatsoever, sharpness and clarity I'd not experienced before, (probably like a Canon 5D MKIII)! But £900 less expensive.
I have a 6D and a 5d iii. The 6d replaced my 5dii and then I bought a new 5diii for £1800 in Feb. I swear by them both. Love using them both equally. I would not think twice about upgrading the 5Dii for the 6d. It's a big upgrade and not a sidestep in my opinion. Technically it may be a sidestep but the best tool that anyone can hand me on a camera is a high ISO and the 6D has the best out tthere! That's just my experience.
Well I bought a video camera, lower end canon, with the change to keep me going until I can get a camera again and get back on my feet. If I had cash now it would be the mkiii and a 24-105. The video camera is new to me and has been interesting learning. Having a zoom and something designed for the job has certainly made a difference. Albeit quality is a little lower but not massively. Here's a test shot in auto. http://www.youtube.com/watch?v=O6dtW7C1Pt0 Not bad for about 300 quid to keep me entertained until cash flows back. Photo option is terrible but I don't really use that. Low light isn't great. But all round a great camera. Wouldn't hesitate on an xa10 as an addition to dslr in the future. I know the sensor isn't as good but you cant beat the ease of use in comparison to rigs and follow focus.
I was thinking to myself 'what a prat going and buying a £300 camcorder' but thought id just have a giggle at how rubbish the quality of the HD video is on it.... So looked at your youtube and all, I can say is fanbloodytastic quality. im sure it has its limitations but for £300 a great bit of kit. I sometimes like using inferior kit.... A bit like getting a speed record ina mini rather than a ferarri
Lol, I actually went there to get the 1200d is it? To keep me going until the cameras back, then I saw it and thought that's not bad quality actually. It's quite surprised me. I suppose it's how you use them too. I'm only learning but I've seen decent footage on phones these days. Ties me over until the pride and joy are back.