I bought the Mk3 back in July/August, now it's been back to Canon, I'm pretty happy with it. Not sure it's 100% but then it's a very complicated camera to set up (more so than the Mk2) and until I've had a proper weekend of heavy duty photoing and chimping I'll not be 100%! However, the feel of it is very much like the Mk2. The menu structures are totally different but you do get used to that!
I'd suggest hiring one for a weekend and go shoot with both, then you can properly decide, I doubt we'll make your mind up! See if you can get a deal where if you buy the camera, they'll discount the hire price off the camera!?!
Cheers,
Carl.