Between these two it seems very simple to me. If you want to shoot always off a tripod, get the RB. If you want to carry it around on a neck strap and shoot handheld, even occasionally, get the Hasselblad.
Of course there are other options in both formats. The Fuji and Mamiya rangefinders are much much more easily hand held in 6x7 as, for that matter, is the Pentax. There are other 6x6 cameras and then 645 if you always crop. I have a 6x6 TLR and sometimes I crop those negs to rectangles and sometimes I leave them square.
I also have a pretty large Mamiya 645 Pro kit and the comment above about it adding up is spot on. With the camera, several backs, AE prism, three lenses etc. it weighs more than my 4x5 Tech III kit complete with three lenses and holders. (My 8 lb. tripod I always take with the 4x5 and not always with the Mamiya more than makes up for that though.) Of course I don't have to take the whole kit. I can just walk around with the camera, one back and one lens and have fun taking pictures. Then again for that kind of shooting I enjoy the TLR even more. Sometimes there are too many choices!