Thanks, y'all! The 15-85 wasn't even on my list, but it looks promising and is well within my budget.
I think I'll end up renting the 15-85, 10-22 and the 24-105 for a weekend so I can play with all three and see what I end up liking best. I'm leaning more toward the 15-85 and the 10-22 now since they're made specifically for a crop sensor and are much less expensive.
The 15-85 and the 10-22 are excellent choices. Combined with the lenses you already own (sell the Sigma
), you will have wide to moderate telephoto very well covered. If you're not in a hurry, wait for canon to have a sale on their refurbished lenses. Very recently the 10-22 was going for $400 which is a silly good price. The 15-85 can be had for around $550. A great site to keep track of all prices Canon is http://www.canonpricewatch.com/