I've used the Pro S many years ago and it's a fine camera. I've never used any of the 645's.
These are kinda on opposite ends of the spectrum, you probably should handle a couple to get a feel for the difference in size, weight, and the difference in negative size.
One thing about the RB's they're so dang cheap you could jump in and if you didn't like it should be able to get your money back.
By the way, welcome to APUG!