I have shot a few events in the same type of environments and I have say that even a 2.8 lens in not nearly fast enough. In clubs people are usually moving about and it is tough to freeze the action. So I'd suggest a 24 1.4 or 35 1.4. The 50 1.4 might be a little narrow, might be okay on a FF. If you have to go with the 24-70 plan on using a fill to freeze some of the action.