I currently shoot both systems, RB67 and Mamiya 645 1000s

The RB feels like a 3 ton studio camera (and it is). It has a huge negative, rotating back, you can change backs mid-roll, lenses for it are relatively cheap and cocking the shutter feels like sliding the lever on a bolt-action rifle. The shutters are in the lenses and are leaf shutters, which synch flash at all speeds. If you get one, be prepared to be asked a LOT of questions if you're shooting in the field with it. People are always asking me about it. It becomes slightly-less awkward with an auxiliary flash like a Metz CT45, sometimes I carry it just to make it easier to handhold. Otherwise, it is my go-to film camera when I don't feel like lugging 60lb of large format equipment.

The 645 has a few advantages too. It's lighter, feels more like a traditional SLR, has fewer light seals, faster lenses with a 1.9 option, focal plane shutter built into the body, the lenses can be adapted to d*****l if you're into that sort of thing and it feels more comfortable in the hands. It still produces a negative vastly superior to 35mm. It also has the option for a d*****l back depending on which 645 model you get. The lenses for it are top notch and you get a few more shots per roll of film.

I find myself reaching for the RB more but I love both systems. They both produce wonderful results and the lenses on both systems are tack sharp, right up there with my Canon L lenses.

If you want the big 6x7 negative but not the awkward feel of the RB you should consider the Fuji GW670 or similar rangefinder cameras. I believe the Mamiya 7 is 6x7 as well and has interchangeable lenses.

The big question has to be "is the size and weight of the RB worth the increased negative size over the 645?"

Absolutely.