The older one has lots of cheap lenses available on the used market. However, it's fully manual, including lack of meter. In my honest opinion, that defeats one of the biggest reasons to buy a 645; you can run around snapping photos with them as if they were 35mm's. One camera you can, on the other hand, do that with, is the Pentax 645. If you don't mind paying a little more for lenses, and can stand the use of inserts instead of film backs, then I think it's an amazing little camera. However, it only flash sync's up to 1/60. On the other hand, there's the Mamiya 645 pro. Great camera. Uses backs instead of inserts, lenses are good, metering is pretty good, etc. The body, however, will probably cost you more than a full outfit for either the Pentax 645, or the older Mamiya 645. Also, the finder in the pentax is much nicer than either of the mamiyas; much brighter view.
I'm in the same market as you rt now. If i can scrape up the cash for the Mamiya 645 pro, then i'll probably do that. Otherwise, I'm personally gonna go with the Pentax. Unless you count Bronica (which you shouldn't lol), the older Mamiya 645 would be my last choice between the three.