If you get the RB, you can still shoot 645 with one of the many available filmbacks that tend to be really inexpensive, and still have the rotatable back and the ability to shoot 6X7 with one of the 10 shot backs. I have not used the 645 cameras, but can say that the RB is definitely a large and heavy camera. Whether that would be an obstacle for the range of shooting you have in mind can only be determined by trying one.