You might look at a 645- a sort of philosophical compromise between the shooting speed and bumblebee compositional style of a 35mm slr and a bigger, slower MF beast like the rb67. There is definitely some advantage to 645 capture relative to 35mm, and there is also a big advanatge (along the lines of what Mark said) to a 645 versus the rb in terms of handholdability. There are a lot of supercute 645s and some pretty bright lenses for them too. The mamiya manual 645 lenses are a steal, by the way. I just bought up a bunch.
You cannot really do available light shooting with an rb, the lenses are way too dark. The fastest lens in the rb/rz lineup is f/2.8, which is the bare threshhold of brightness if you are into nighttime street/club stuff.
When I was getting into MF I must have gotten just about everything from KEH to try out. I settled on the rb, but in addition to that, I happily shoot a mamiya 645 and an F100 and an OM1 and some XAs. Different tools for different tasks, of course. I recently deployed the 645 for some dark club stuff at 3200 in colour, and was quite pleased.
Along the lines of what DBP said about F5 "overkill," I agree with that- an F5 is something that you shouldn't get unless you are crystal clear what it is going to do for your shooting. The thing is weathersealed for 'nam and the AF probably develops enough torque to run a ceiling fan. The F5 is not overkill for
some people... but that you are unsure whether it is right for you, means that it almost certainly is overkill. It is chick magnet though, that is for sure. So choose wisely
