I own a Contax 645, but have used every major MF camera so hopefully I can help.
I think as far as AF speed and accuracy, both the Mamiya and Contax are fairly equal. The Mamiya is quite noisy wheras the Contax is not. For me the tie-breaker in /this/ instance was the manual focus. The Zeiss lenses are far superior in-terms of build quality, and they have just the right amount of resistance. Things genuinely do pop into focus compared to the Mamiya which is a little 'thin' and 'tinny', making it difficult to operate.
While I have used Mamiya on several occasions, I don't own one so I can't say if the following problem applies to both Contax and Mamiya....or just my particular Contax 645. Sometimes focus can be a little-off and is actually focussing slightly higher than the AF point.
The AF confirmation light is basically always right though it seems....Has anyone else had this issue?
Ras