Now that I have my first full fledged wedding job for the year now done, it would clearly be the 1DsMKIII. (Used a 1DsMKII since it launched, until the end of wedding season last year).
Reason 1: the operational aspects.
The MK3 camera is faster to use, the controls have been simplified and the menu is simpler to navigate. IMO, there is one main reason to use an AF DSLR ... Speed!
The MK3 is lighter (thanks to the smaller LP-E4 battery?) ... no small issue when you have camera in hand for 8 straight hours. Plus, I don't have to pack/carry the big ones like with the MK2. The MK3 has a sophisticated battery monitor that even tells you when it's time to Calibrate. I shot 8 gigs of images to 2 cards with the LCD continuously enabled, and never turned off the camera for 6 of the 8 hours ... the battery monitor reads 67% full.
The MK3 LCD is larger, which isn't really a reason to select it over the MK2 except unlike the MK2 the MK3 has Live View where it does help to have the larger LCD. Live View is a VERY handy tool for composing and checking focus when on tripod (in my case, doing group wedding formals.) There is a menu option to include a grid display.
The MK3 has Highlight Priority... again, an advantage for my line of work where the key subject is wearing all white in a sea of black.
The MK3 has dust deletion ... which from my experience with the 1DMKIII works. Important, since I change lenses frequently during the course of a wedding, including outdoors. After 1 year of use, I have not had to clean the sensor of the 1DMKIII.
There's a host of other operational improvements, but that's enough for here. BTW, zero AF issues.
Reason 2: the image aspects.
This is where it gets dicey. The MK2 was no slouch. However, in practical use either is the MK3. The MK3 simply gives you a little more to work with if you crop or enlarge. Of 400+ images done with the MK3 I cannot detect any obvious "softer" feel compared to the MK2. What I can detect is less tolerance of underexposure compared to the MK2. This may well be my lack of experience using this new camera and I still have to tweak it more. So, this is TBD yet.
Here are my suspicions concerning image quality with the MK3:
I think some Canon lenses are visibly not up to this camera's ability. Here is why I think that: I recently purchased a Zeiss N24-85/3.5 with a fully automated Canon mount from a forum member ... a lens I am very familiar with having shot it on Contax N cameras, both film and digital. It wails on the Canon midrange Zooms ... all of them, even if they're stopped down a bit. In fact it out-performs many Canon Primes at some focal lengths. Wedding images shot with that lens stand out among those shot with the Canon lenses. NONE of them look soft ... most likely because of the Zeiss contrast in conjunction with this sensor and AA filter.
What I need now is to get my hands on a converted Zeiss N17-35/2.8 and a N85/1.4 to see if this holds true. BTW, the N Zoom focused just as fast or faster than any Canon Zoom even in low light. Faster than it did on a Contax N camera (probably because of the MK3's more advanced AF, plus focus assist when using an EX flash or ST-E2 alone in the hot shoe.