The lens selection for RBs and RZs is very broad, they were among the most common MF cameras and the lens availability as well as the availability of other parts is fantastic. The mamiyas have lenses as short as 50mm, which is probably equivalent to roughly a 22mm to 25mm lens on a 35mm or full frame digital. The Pentax does go a little shorter, to 45mm, so if you want the widest possible lens, that is an advantage.
I also find that the mirror on an RB is very well dampened, I personally haven't had issues with shake. Both the RB and RZ are full system cameras so you can use a waist level finder or a prism. The RZ also has some more modern niceties available such as an AE finder. I usually end up choosing the finder based on whether I need the camera up high, in which case I use a prism, or low using the waistlevel. When I first started using a waistlevel finder it was a revelation. The whole composition thing is different when looking with both eyes at the ground glass rather than looking through the viewfinder with one eye. I totally love it and it is one major reason I chose the RB. With its revolving back, one can compose for either landscape or portrait orientation with a waist level, which is just great to have.
I know that the Pentax 67 is a fantastic camera, but I really love the way the RB and RZ systems work. If I were to start over, I would get an RZ because for whatever reason their lenses seem to be better values and are newer and generally considered to be better than the older RB lenses. (RB K lenses are based on the RZ glass and are great, but more expensive than either RZ lenses or RB C lenses.)
As for rental, I think it is a great idea, but if you do not have a local rental place I would recommend getting the camera you are most likely to want from KEH and then returning it if it is not right for you. The lost shipping fee and return shipping cost will likely be a lot less than the rental cost and shipping from an out of town rental place. KEH offers returns for 14 days from receipt, so that gives you plenty of time to try something out. Their prices generally range from decent to extremely good.
One other massive advantage of the SLRs over the Rangefinders is cost. With any of the Pentax or Mamiya SLRs you should be able to get a full kit with two or three lenses for less than the cost of a Mamiya 7 with one lens.