Brett, the 24-70 f/2.8 is a great lens for concert photography, and you can save a ton of money to pick up the Sigma over the Canon version. I've shot with the Sigma version for over a decade and have sold more images than I can remember from that lens. Since you have a cropped body, Sigma just came out with a 18-35 that opens up to f/1.8 that would be great for low light photography
Also, you can get pretty good results depending on which body you have by bumping your ISO way up which will compensate for a slower lens