Hi, I am in the same position as you, I want to upgrade from my 550D to a full frame, I looked at many possiblity, i even made a spreadsheet with pros and cons for many cameras, from latest nikon to sony a99, new gH3, blackmagic, fs700, 5Dm3, 6D
All this time making a spreadsheet just to realize that I'm stuck with canon lenses, so no nikon or a99 for me, I don't want to bother with an adaptater either, so no FS700 or GH3 (both came out good in scoring in my spreadsheet but GH3 isn't a fullframe and not great at low light (if we compare it to a 6D/5Dm3) and the FS700 is just way to expensive so no slow motion for me)
So finally I want to stick with canon, and magic lantern, so I have to decide between the 6D and 5Dm3, (1D-X is too expensive also and Magic lantern might not come out on it anytime soon since so few people have it)
I don't include the 5Dm2 because it's doesn't match up on low light performance, and this is very important for me, I want to reduce as much as possible my use of noise reduction filter.
My biggest disappointement with the Canon is that we can't shoot at 1080p at 60fps! The Gh3 and a99 can!
I think I'll go with the 6D, I watched many reviews, and its quality is great and some reviews even show that 6D is better at low light than the 5Dm3, and the points that bother most people don't bother me :
- Not so good AF (i don't care I use it for video)
- slow photo fps: 4.5fps (i don't care I use it for video)
- No headphone jack ( i use a H4n with a cable that allow me to record its good sound in the 550D and at the same time connect use headphone)
- Only one card slot (it's really not a big deal!!)
- moiree (well.. Too bad, but it can't be as bad as the 550D)
- smaller screen (kinda bad, but I have an external LCD)
I'll wait before bying it, I'm not on a hurry, I'm wishing Canon release a camera with autofocus video like the a99 and the Gh3, and the most important for me would be 1080p at 60fps (or more,we can dream...)
I wrote a lot about my opinions, I don't kow if that can help you, I hope it does.