I have the T3i and the 17-55 2.8, it takes a special circumstance for me to take it off and put anything else on. I use it for the vids, stills, landscapes, etc. I went through the 15-85 upgrade (want it? it is for sale) and found that pretty much anytime indoors it was too slow - if I am going to be outside and can only take one, it MIGHT remember to take the 15-85. I also have the 70-200 II, and have a small bag that I take that fits it, the 585 and a couple gadgets, straps a mono/tripod to the bottom and this covers just about any day trip I might take.
24 is not nearly wide enough for a crop body and if you think about it the 15-85 covers the same effective range on a crop body vs. full frame as the 24-105. The 17-40 stops short in both reach and speed. You will quickly forget the few extra dollars spent, unless you don't spend them and can't get the shot cause you have a slower, shorter lens... Then by gosh you will remember the decision and most likely apply say or think something not best expressed out loud.