If you really want to shoot 6x6, carrying the extra bulk of an RB-67 does not really make sense. Unless your camera is going to stay in a studio of course.
True, unless, of course you actually tend to print (or have clients that use) rectangular images. I have several folders, some TLR's and both Hassy and RB systems, my Hassy system has been pared down to a 500EL and a 150 Sonnar, otherwise I shoot the RB. Traveling light is possible, a 65/127/180, body and 2 back outfit easily fit into a backpack, even a larger camera bag (I use an ancient Tamrac pro 12, now called 612, I think.
The RB is larger, but not really that heavy, after all , it's mostly an empty box. I found myself reaching for the RB almost always instead of the Hassy, so I ended up selling off most of the outfit and keeping just the one body/lens combo that I do use. There is a lot of advantage to real estate, 50% more usable negative is a big advantage when enlarging.
There are other options, if you know you want to stay with 6x6, a used 500C/80 C/A12 back can be had for $500 or so, if you're patient. (the only advantage/difference a C/M has over a C is interchangeable focusing screens, how often do you really think you'll be changing it? I had mine changed once in 20 years, mostly due to the fact that it got pretty ratty because I tend to mark up the screen with a grease pencil for still life work) Yashicaflexes are highly regarded, the Mamiya C series are nice, but if you think an RB is heavy, wait til you pick a C330 up, it's at least as heavy, if not heavier. I used a Rolleiflex all through high school, but at this point, with the scarcity of good, reasonably priced repair persons, it'd be an expensive option to maintain. A Ciroflex would be another option I've heard some good things and seen fantastic images from as well.
erie