It's not a huge jump, I like the 645 format and used it extensively. In terms of print quality you'd need to look closely to see the difference and the portability and easy of use makes the smaller 645 camera greatb to use.

Back in 1986 I looked at moving up to an RB76 from my Mamiya 645s and in the end decided as I nearly always used a tripod I'd shoot 5x4 instead - similar weight etc.

Ian