i was in the same situation as you a month ago.
I was in the market for a 645 slr, i needed something with ttl prism, handheld friendly, nice lenses and fast shooting flow (AF and motor wind)
After reading tons of reviews on the net i was quite stuck between mamiya, pentax, contax. So i went to a local shop and ask them to allow me holding and playing with them, i tried bronica and mamiya (no AF) years ago and i knew they werent for me.
Obviously any of these systems are excellent, so i needed to figure out small differences between them. Apart from Pentax not being able to change backs, all the rest is quite the same.
Conclusion, I'm in heaven with my new Contax 645, why? optics are BETTER, you dont have that bokeh with mamiya or pentax and the user interface is perfect, shooting experience is awesome, seriously!
I think that you should try all three and decide for yourself, my only regret with contax is that it behave and feels like a very expensive camera, nothing that i would carry all day long (quite heavy aswell).