Keep your 17-85. You seem to be satisfied with it.
If you get a full-frame camera in the future, then buy the full-frame lens at that time. There may be new ones coming, or you might find a deal when it's relevant. But why spend money on a lens that you apparently don't need, for a camera you don't own?
Agreed. The 17-85 is a solid lens. The 15-85 is a step up, but probably not enough to justify the cost. And if you ever go full-frame later, that's money down the drain.http://www.juzaphoto.com/article.php?l=en&article=32
Alternatively, you might wait, save up the extra few hundred bucks, and buy a 6D/24-105L kit. That will do far more for your image quality than any lens upgrade by itself, and it's not a lot more than the eBay prices for the original 24-70 f/2.8 lenses. And the 24-105 is also sharper than the original 24-70 L f/2.8 even when stopped down to f/4, according to LensRentals.
So IMO, the only reason you should even consider the original 24-70 f/2.8 L is if you shoot a lot of sports and really
need the f/2.8. Maybe not even then. The increased ISO capabilities of a full-frame body will usually do a lot more for you in terms of usable low-light performance than a single extra f-stop on the lens.