The answer totally depends on:
- The light levels in your gym (f/2.8 is often not fast enough)
- The light distribution in your gym (seriously uneven lighting may require flash)
- Your shooting location (the closer you are the less FL you need)
- Your shooting skill (less skill requires less FL)
- Your system's (camera + PP) noise performance.
In my case the best choice (1D-IIn, medium gym light levels) is the 85 f/1.8. 2nd choice is the 50 f/1.4. I shoot at ISO 3200, 1/500, f/2.2. f/2.8 Zooms drop me to 1/250 - too slow (we can discuss the extra light loss shooting wide open in another thread).
I shot with the Canon 50 f/1.4 and got good results with it. AF was fine (on 40D) though not quite as good as the 85. It gave a nice angle of view for under the basket shots and baseline drives toward the bucket where the 85 was often a bit too tight. My 135L gave some great results too but was too tight for most of the close-in shots. It was fine for pics of the point guard setting up the play though.
When I switched to the 1D I found the 50 to be a bit too wide and the 135L became a bit more useable. The 85 simply rocks on this body for basketball.
The issue I routinely have is DOF. At f/2 or wider, DOF is shallow enough that I get shots with the ball (frame center) in focus but the player's face slightly out. My customers don't seem to mind but I can see the difference. At f/1.4 the DOF is too shallow to be workable at focal lengths above 30 mm or so. Also, lens sharpness falls off at f/1.4 so images look better at f/2.
My local press guy uses Nikon with 80-200 f/2.8 exclusively. But he has to use flash most of the time and confesses he wishes he had a fast prime. Alas, as a working pro he has no money so he has to live with what he has.
F/2.8 zooms I've tried (24-70L, 70-200L) have been too dark wide open to support the required shutter speeds. The 24-70L also suffered from a lack of sharpness from 50-70 so it fell from grace.
