i had a bad experience with the RB pro-s, got it in HS and once i got to college, i learned all the times on the lenses were way off. i got an RZ and really liked it, i would recommend it. do get the newer lenses (with 1/2 stop fstops indicated on the lenses... i assume they are the new ones), the RB is alot older, the new KL lenses as the other guy mentioned appear to be just as good as the RZ lenses but i havent used them.
the Rb has one lever for the mirror, another for the film back advance, the RZ just has the first which does both functions. i think the RZ is from like 1990 and the pro/pro-s is from the 60s/70s. the RZ Pro II has a micro focus knob and 1/2 shutterspeeds, something the old RZ or all RBs lack.
on the used market, both systems are very cheap, and the price difference isnt much. great deals from keh.com.