The camera technically does not have a revolving back until a revolving adapter is mounted. (The RZ does, however.) However, 99% of them come with a revolving back adapter in place.
On the RB, you have to cock the shutter/lower the mirror and advance the film separately. On the RZ, the same motion used on the RB to do the former also does the latter via a rotating linkage.
RZs are so cheap now that they are an option too, if you are looking at later model RBs. They will mount RB lenses or RZ lenses (but an RB will only mount RB lenses).
RZs will not accept a 6x8 back, and as I mentioned already, the revolving part is built in to the camera body, not a separate adapter. Therefore, they won't take all the cool stuff other than Mamiya RB/RZ rollfilm backs, such as sheet film holders, plate holders, Graflok accessories, Mamiya Press accessories, etc. (though it will take a Polaroid back).
The RZ is a mildly simpler and smoother camera in use, but it lacks certain benefits of the RB.
Personally, I think the killer kit is one RZ and one RB, with all RB lenses.
If I had to go for just one, I'd go for the RB first...because it takes a 6x8 back.