I'm new to photography, so don't listen to a thing I say, BUT...
I guess the Sigma 28-70mm is cheap enough to give a shot. But you will find yourself wanting to replace it with better glass before too long, I bet.
I just got the 10-22mm for my 450D and used it for the first time when I was in Chicago. It is an awesome lens. I love shooting wide. The 24mm is not nearly wide enough for me if I could only have one lens for walk around. I'd hold out for something that was down to 17 or 18mm. Or just accept the fact that 2 good lenses are better than one.