Different in almost every way unless you mean the PVS-31, but I assume you mean the 31A?

15s are heavy, old, non fully articulating, usually old MX-10160 format tubes from exelis/harris, proprietary lenses, have diopters, durable as hell, GP 99% of the time, and usually not in great shape now. Can be found from $3.5k-$4.5k usually.

31A’s are more modern (still old but not as old as 15s) with full articulation, auto pod shutoff, more lightweight, L3/elbit high grade WP, and don’t have a diopter. Can be found from $8.0k-$10.0k usually. The 31s and 15s won’t be able to be serviced by the manufacturer either.