comparing 15-85 and 24-105L on crop sensor, based on a single lens usage, the former will top out due to its more useable focal range.
since you say you don't need more than 15mm after try out the UWAs, then the 15-85 is for you. IQ is comparable with 24-105L, maybe just lacking in a bit of contrast(can be fixed in pp anyway).
since you say you don't need more than 15mm after try out the UWAs, then the 15-85 is for you. IQ is comparable with 24-105L, maybe just lacking in a bit of contrast(can be fixed in pp anyway).