I'll second Nate's recommendation - the EF-S 17-55mm f/2.8 IS USM is an excellent lens, fast and with great IQ, image stabilization, and a very useful focal length especially for indoor shooting. You will want something as wide as 16/17mm on your body; 24mm isn't wide enough indoors. With the EF 16-35mm f/2.8L II USM, most of what you're paying for is sharpness out to the corners, which on your 1.6x crop body is not useful since the corners of a FF image circle are cropped off. You're also paying for weather sealing (when you add an 82mm filter, and good quality filters at that size aren't cheap), but indoors that's not a need. So, unless you're planning on switching to full frame in the near future, in your place I'd get the 17-55mm f/2.8. That lens is on my camera most of the time.