I'd take the 12 ga based on ammo alone. I find a much wider variety of loads available on the Internet as well as in local stores. There are reduced pellet loads, reduced recoil loads, sporting loads, law enforcement loads, less lethal loads, etc. For the utmost in availability and versatity the 12 is hard to beat.
I'll even throw in another plug for the Mossberg. I've shot my personal one since the early 1990s as well as the military version of the 500 and 590 in the Navy. I also have an 870 and find both guns to be very dependable.