I suggest TS to rent the 24-70 first. The 24-70 may not be wide enough on crop. I rented once, and went to shoot events. The contrast and color is superb, which I didn't have to do much PP in it. Just that this lens, both 24-105 and 24-70 might be too heavy, not too well balanced on the 550D. However, TS may also consider the 17-55, wide enough, lighter and also superb in it's color and contrast.