I have both systems and I am extremely happy with both. IF...you want to handle and use the camera more like a 35mm then you should go with the M645 if you plan on working from a tripod mostly the the RB is an excellent choice.
The negatives from the RB are beautiful because they are big...but compared to a 35mm so are the 645 negatives.
the other advantage of the RB is interchangeable backs...really nice. The back will rotate to change fro landscape to portrait without having the turn the camera. Also the bellows allow you to focus very close. Drawbacks with the RB could possibly be only 10 frames per roll of film opposed to 15 with the M645...personal preference there.