If you shoot handheld get the 645.....personally, I'd look at both the Pentax and Mamiya 645. I think the Pentax is easier to hand hold with the grip, but the Mamiya is smaller. The quality of the image isn't all that different from the 6x7 unless you print pretty big.
I have a Hasselblad and RB67 system. I also had 645 systems in the past. I love my RB and I love my Hasselblad. Never liked 645. I have no problem carrying the RB67 with a grip for hours and using it purely handheld, I can easily shoot it down to 1/15. Ok, it can get a bit heavy after 4-5 hours but when I get the film back it is superb. It also makes my Hasselblad look small and light. Somehow I tend to pick the RB more often than the Hasselblad nowadays.