18-135 STM vs 15-85
It all depends on if you were "lucky" to get a good (properly adjusted/calibrated) copy. Canon QC is very "patchy", to say the least.
Both lenses, on any properly adjusted/calibrated EOS body, can deliver stunning images. I have both lenses but am mostly using 18-135 at the moment - on 600D body! On the screen and on prints (up to A3) the images don´t look "worse" than from 5DMk2+24-105/4LIS combo.