Welcome Tack Sharp,


I would go with the EF-S 15-85mm f/3.5-5.6 IS USM of the two choices you listed. I have the EF 24-105mm f/4 L IS USM, but on a APS-C 1.6 crop Canon, such as the T3i, 24mm is the equivalent of 38.4mm on a FF body. Personally I have the EF-S 17-55mm f/2.8 IS USM because I needed the constant f/2.8 for what I do. If you are not using it in low light without a flash the EF-S 15-85mm f/3.5-5.6 IS USM is a very good choice. Since you have a T3i, a fairly new body and are looking to upgrade from the kit lens, I am making the assumption you are not looking at FF anytime soon. If you were thinking of say a 5D II purchase soon, then the the 24-105 would merit more consideration. If you have not done so yet, I would recommend you read Bryan's excellent reviews on both EF-S lenses.


http://the-digital-picture.com/Reviews/Canon-EF-S-15-85mm-f-3.5-5.6-IS-USM-Lens-Review.aspx


http://the-digital-picture.com/Reviews/Canon-EF-S-17-55mm-f-2.8-IS-USM-Lens-Review.aspx


Fair warning though: good Canon glass is like Lays Potato Chips -- one is never enough.


Welcome to the addiction,


Chris