And yep, the SD backs are convenient, but if you get a deal on a Pro-S back, go for it - if it has light leaks, google Jon Goodman for a seal kit. Re-sealing them takes less than an hour from laying out a towel to cleanup, and (with his instructions) it's easy and satisfying, and you're done for another decade or two.
He has a kit to redo all the foam on the RB as well, including the mirror shock absorber. It's a bit more work, but if you're even vaguely handy and like the old gear, it's very satisfying to bring an old girl back to her prime, and it's really a once-a-decade or longer operation.
Skip the original "pro" backs (and the non pro-S cameras), there are too many great features that came with the Pro-S (if you don't go RZ). Many used sellers say "Pro, Pro-S, Pro-SD" on their back listings since they'll work on all the cameras. You can spot a Pro-S back by two things: the Chrome "Pro-S" logo on the shell-door, and the double exposure lever under the winder. SD backs look more squared off and "modern".