Of the two I'd go for the Canon EF-S 15-85/3.5-5.6IS USM
. It's quite a bit wider, plus it's overall a better lens. You already have 70 and up covered, so you really don't need to go to 135.
I guess another alternative would be to keep the Canon EF 28-135/3.5-5.6 IS USM
and add the EF-S 10-22/3.5-4.5 USM
to your kit. If you want dramatic wideangle shots, that would be the way to go. 15mm is wide on APS-C, but not superwide.