Upload & Sell: Off
Before purchasing the 15-85 I tried the 18-135 first. The reason I went with the 15-85 was because I felt the 18-135 was not quite as sharp on the wide end. It was great between about 50 and 135mm but I wanted something a bit better on the wide end. The 15-85 does does the trick. The fact that it is a little smaller helped too.
The 15-85 is fatter and heavier, while the 18-135 STM is longer, skinnier, and lighter-
But the 15-85 is also, as you say, sharper on the wide end, and sharper across more of it's shared focal length range with the 18-135; and I did enjoy the wide end of the 15-85 on my 60D.
Makes the choice difficult, in that sense- but for it's intended uses, I'd expect the 18-135 STM to be more useful, especially if bought in a kit/from a parted kit. Retail, as with the 24-105L, is quite ridiculous.