For a crop camera I would use a Tamron 17-50 2.8, or Canon 17-55,or a sigma 18-50 HSM on a Nikon. Also a 70-200 2.8 IS.
I usually start with a base ISO of 800, going higher depending on my shutter speeds and the lighting.
Primes would work great except for crowded areas where it is difficult to move around. In larger clubs, the stage is bigger and its harder to move around even in the Photo Pit. for example: It sucks if you are shooting the drummer, upstage, with a 135mm then the bass player, downstage left, starts doing something cool right in front of you; there is no time to change to a wider lens to get the shot. That is where the zooms are very handy. To me a single 50mm is too limiting. Using to bodies and having a 20mm and a 50mm, or a 50mm and an 80, or 100mm would probably yield better results.
In bar settings Primes tend to work better because the lighting often sucks so you need faster glass, an you are generally more free to move around to compose a shot.
Its really a matter of matching your lens to the venue; there is no one answer.
In one venue the lens of choice and in some bars any might be a 70-200 2.8, and in some bars anything longer than 50mm is far to tight. I have shot in some bars where most shots were taken at F/2 because it was so dark. It really all depends on the situation.
What gear go you have now? A 2.8 wide-normal zoom is nice to have in general for most types of photography, and you can never go wrong with a fast 50mm. That would be my starting point and then adding things based on your needs from there.