I shoot a 5DIII (previously II) for the faster and less-well-lit parts of weddings and have a lot of respect for the camera. The odd fact is I could show you a Phase One H25 from 2003 which will match or exceed the 5DIII on image quality in nearly every category. Granted the H25 was only good at base ISO (50), could only shoot tethered, was limited to 60 second exposures. But it's color rendering, tonal smoothness, and overall image quality still edge out my use of a 5DIII. As just one small pet peeve, Canon STILL cannot render a skin tone transition in dramatic lighting without a strong color break between highlight and shadow (regardless of which raw processor, setting, etc you use).
It comes down to, more than anything else, the priorities of design of a system. Phase and Leaf, when facing a decision between image quality and [cost, speed, features] choose image quality every time. The sensors selected, the way the data is read off of them, the color filters and IR filters chosen, the internal electronics to convert the analog signals to a raw file - they are all selected first and foremost for image quality.
So yes, I think you'll be very pleased with the image quality, color, and tonality from any Leaf Aptus or P/P+ back. For shooting people I'd suggest a 30+mp back to avoid issues with moire. Likewise, if you like to shoot wide open on your Canon I'd suggest a larger-sensor digital back like the 33mp Leaf or (Aptus II 7) or 39mp Phase One P45+. They will exaggerate the look of wide open shooting. A Phase One 150mm D shot wide open on a large sensor is a really beautiful thing.
Likely others in this thread will get bogged down with numbers or numerical measures of image quality. Someone will mention the D800 and someone else will inexplicably mention the Fuji 680. Trust me when I say I can go toe-to-toe with all that, but it gets tiring. Suffice it to say I've never, not once, seen some one walk away from a hands on experience with a Leaf Aptus II or P/P+ with disappointment regarding the image quality at low ISOs. People do walk away sometimes because the high ISO is not fast enough, or they cannot afford it, the LCD isn't good enough (prior to the IQ/Credo - the LCD of which is stunningly good) or because they needed something with a higher burst frame rate, or video. But not image quality. So if you think medium format fits your style (e.g. you don't shoot fast paced action in low light) and you're curious you should really try it for yourself. Dealers can arrange test files, rentals with credit towards purchase, in studio demos, or remote video chat meetings to that end. Note of disclosure, this is a selfish statement, since my company Digital Transitions would be glad to be that dealer for you.
The other factors here are the lens selections available in medium format, both for uber-sharp modern-look lenses like the Phase One 150D/2.8 and for classic sharp-but-not-clinical lenses like the Hassy 110/2 to the notably-soft-but-still-cool-looking Mamiya 80/1.9.
All that said, image quality is not the only criterium on which to base a camera. Here is a list I posted on a similar thread regarding Nikon dSLRs vs. MF (I've tweaked it from that thread for applicability to your question). It is a partial list of non-image-quality related criterium in which I think medium format has a big advantage:
- large and bright viewfinder
- Flash sync speed with standard strobes rather than dinky flashes (up to 1/1600th, some back/lens combos)
- More tactile lens response when manually focusing (large focus barrel, actual lens gearing*)
- aspect ratio (some prefer 4:3 or 1:1, especially for verticals)
- waist level viewfinder (some bodies)
- ability to shoot verticals without rotating camera (some backs)
- ability to shoot film with same system as digital (some bodies)
- ability to crop a usable vertical and horizontal from the same frame (on higher res backs)
- ability to use on specific legacy cameras (some folks just plain love Contax, Hassy 500)
- ability to use on tech cameras for your occasional landscapes
---- rise/fall/shift/swing/tilt on every lens (if IC allows)
---- fully mechanical/traditional shooting
---- absolute best glass, period
---- ground glass (some prefer it regardless of other options)
- less frequent updates required (we still have many happy studio shooters using H25 backs users, don't know many happy Canon 1D shooters)**
- longer software support (original Phase One Lightphase from 1998 is still fully supported tethered in OSX 10.7 and Capture One 6, while the Canon 5D from 2006 isn't even officially supported tethered in LR4 or EOS Utility in OSX 10.7, nor 1Ds II in Windows 7 64 bit)
- consistent shooting speed; most of these backs can hold it's frame-rate for dozens of frames (if not forever) with a fast CF card, any Canon/Nikon can shoot much faster in a burst but unless you restrain yourself you can easily hit a buffer and the camera won't fire when you think it should. Such a digital back will be slower (around 1.2fps for the 40mp model) but it is reliably consistent - you know when you can shoot next and can develop a rhythm.
- larger bodies (for some this will be a big negative, but for others their hands are simply too large to comfortably use a camera like the 5D3, even with the optional vertical grip)
