A few thoughts:
Forget about IS. It really won't do anything for you. You need as fast a shutter speed as you can get, not the ability to shoot at a slower one. IS will do nothing to freeze the action on stage.
If I'm shooting in the pit, I'll use the 70-200mm f/2.8L on the 6D, and the 24-70 f/2.8L on my second body (either the 40D or the 5D). Someone mentioned a 50 mm f/1.8. That's okay, although I've found it focuses slower than I'd like. The f/1.4 version is a better choice. If I'm shooting from the soundboard, I've settled on the 6D with the 100-400mm f/4.5-5.6. In and of itself, it's not the fastest of lenses. Coupled with the ridiculously good high ISO performance of the 6D, however, it's a perfect option.
Unless you're doing tour photographery, you won't be shooting from the stage, so I'd recommend not basing your buying decisions on that factor.
I really want the Canon 24-70, but I believe that's out of my price range. Do you have an opinion on Sigma's 18-35 ?
I can't speak to that lens in particular, as I have no experience with it. I do like Sigma lenses, though.
That said, that focal length would just be too wide for me. I take an occasional wide shot but, like anything, too much of anything is boring.
What's your price range? The 24-70mm f/2.8L II is right around $1,800.00, but the 24-70mm f/2.8L IS is about $800.00 less at
B&H. The Sigma 24-70mm f/2.8 is $100.00 less than
that.
Depending on how far you can stretch your budget, you could get the Sigma 24-70mm f/2.8 and the Tamron 70-200mm f/2.8 for a little bit less than the cost of the Canon 24-70mm f/2.8L II.
