Starting point -- I have three bodies: 5Dmk3, 5Dmk2, and a 30D, which I rarely use. I do this for a living.
I see no new bodies coming soon and don't like the sound of (and the reviews of) the 6D. My desire to get more flexibility and back-up in my bodies was accelerated yesterday when I discovered that my 5Dmk2 shoe doesn't work! I have an evening charity event I am shooting Saturday night and was testing everything. There are few rental opportunities in my area but I was lucky to find a mk2 I can rent for flash backup.
I am interested in opinions of the trade-offs for a third FF body between another Mk2, a 1DIIIs, and I guess I'd have to consider another (used?) 5Dmk3.
I should say I am disappointed with the focus ability of the Mk3 in certain conditions so am leery of adding another Mk3. Canon can't duplicate the conditions in which I see difficulties in their repair facilities; Canon and I have gone around and around on this already. Leaves a bad taste in my mouth about the variability of the Mk3 bodies.
The 1DmkIIIs would be used for product work, food, landscape, portraits, architecture, tripod work in general, when I have the luxury of lower ISO. At or lower than 400 is my guess, and often using lighting. I think I would want to shoot high-speed-sync in some real estate or architectural work to balance windows (HDR still problematic for this in some conditions.) Further, I read that the skin tones are good on this sensor. Would shoot this body tethered a lot.
Did I read about focus problems on some of these?
I *don't* get asked to shoot sports.
I get the "old technology" epithet, but we used to buy a couple of used Hasselblads and their lenses and use those for YEARS in billing good day rates. And in a couple of years there may be another Canon to consider. I have to say that the lack of extra DR in the Mk3's low ISOs (over the Mk2) was a big disappointment.
This sounds like a lot of complaining but I am willing to work with the gear as it is... just find the right mix for me. I have never used a 1D.