mamiya 645 af / Autofocus MF

hi,
i'm searching for some input about autofocus MF.
i have already a perfect setup for 6x7, mamiya rb, 3 lenses, 4 backs. But i realized that i need something to shoot hanheld and very fast, something useful on the field. I've already done it with my mamiya, but doesn't matter how fast or hard i try, it's not a proper field camera for fast work.
So, i'm planning to purchase something like pentax/mamiya/contax autofocus series. I need it to be fast, relatively light, and pretty good at metering.
It seems that any of the chosen will serve to my purpose, but i'm looking to some advice.
Does anyone have experience in mamiya 645 AF series?, doest it matter AF or AFD?? is hard to work with the mamiya and use the 80mm f1.9 at manual focus, is the meter or camera full funcional? i do it all the time with 35mm (autofocus body-manual lenses). There isn't any rental facility around so it seems that i have to make my decision based on pure reviews. About lenses, i'm pretty sure that all systems are terrific but, any big diferences? contrast? fast AF?
Thanks in advance

I have the AFD. Reason being, I can upgrade and get the digital back at some point. Otherwise, I think there is little to no difference between the AF and AFD that I am aware of. Sorry, I haven't tried to use MF lenses, so can't address those questions.

upgrading to a digital back it's a defenitly no no.
Based on specs seems that afd has aswell a special metering mode.
I'll keep searching what else can i find.

This FAQ will answer at least one of your questions:

http://www.mamiya.com/customer-service-faq-645afafdafd-ii.html

what a shame, no automatic stop down with manual lenses