bdunbar79 said:
I think in your particular case the 24-105L is the clear winner. If you follow the suggestions of the other zoom lenses, you're stuck with APS-C and can't go FF.
Except you can always sell the EF-S lens if you upgrade...and in the mean-time (which if he can't get another L lens for year, probably means at least a year if not longer), you've got a lens that is better suited for the body he has. 17-55 on APS-C covers landscape through the early end of portraits. The 15-85 would cover the full range he'd want, but, I'm not sure its worth the sacrifice of the extra stops of light.
Get a used 17-55, they go for $750-800 on the used market, and if you switch bodies, sell it for about the exact same price. Or, just as likely, find someone with the 24-105 and trade with them.
The difference between 17mm and 24mm is pretty significant, especially for landscape work. And the difference between f/2.8 and f/4 can be significant in indoor lighting. So why pay as much, or more, for something that would limit most of your photography?