Aperture really was my first thought too. At an indoor venue, I'd never want anything narrower than 2.8. Granted, I read this thread on my phone while walking, so may have missed previous discussion of this, but I usually prefer my 85 1.8 @f/2 to my 70-200 @2.8 if I'm indoors. Opening up more will give you better background blur and a faster, action-stopping shutter speed. I know everyone always says "a stop isn't that much", but in terms of stopping action, 1/1000 is much better than 1/500. So I'd go as wide as reasonably possible, and definitely recommend at least 2.8 if not faster.