If you want a normal to telephoto zoom on your 50D, the 24-70 f/2.8 is the best choice. I've shot a few weddings with the 20D and the 24-70 f/2.8 and it worked great, but I really missed having a wide angle. You can


For a wide-to-normal zoom, though, the 17-55 f/2.8 IS is the cream of the crop. The image quality is a little better than the 24-70 f/2.8 L, but the biggest difference is the field of view and image stabilization. You couldn't ask for a lens with more resolution or contrast. Build quality and focus ring is lacking, though. If you don't need IS, you can get the Tamron 17-50 f/2.8 for less than half the price.