The extra 9mm at the wide end of the 15-85 counts for a lot more than the missing 20 at the long end, IMO. I'd get the 15-85.
24mm on a crop body is equivalent to 38mm on FF, so it's not wide angle. You can always crop the image a bit, but sometimes you can't back up. You can't just look at the numbers - even though with the 24-105mm you're giving up 9mm on the wide end for 20mm on the long end, the 9mm is much more significant.
Personally, I find 24mm on APS-C can often be too narrow indoors. As stated, if you have an ultrawide zoom for crop (10-22mm, etc.), that makes a nice combination with the 24-105mm, as long as you don't mind switching lenses. I also used the 24-105mm as an outdoor walkaround lens on my 7D when it's raining, because the 24-105mm is sealed - not a benefit when using a T1i.
Personally, I'd recommend the 15-85mm - it's definitely a more versatile lens on APS-C. I actually prefer the faster aperture of the 17-55mm f/2.8 myself.
I'd also suggest that you can answer this question yourself - I presume your kit lens is the 18-55mm (or some other 18- zoom), and you've shot for three years. Look back at the EXIF for your images and see how many were shot wider than 24mm; if it's a lot, get the 15-85mm, and if it's few, get the 24-105mm and keep your current kit lens for times you need wider than 24mm.