i haven't had much experience with Remington [only firing their 870 twice], but i love my 500a. now if Mossberg carries the quality of my 500a, into it's other series, then you'd be better off with the Mossberg. Also,from what i've seen, their are more Mossberg parts laying around on the internet then Remington. Now the Mossberg is tough. I mean tough. One of the shells my little brother fired from it had way to much powder in it or something, because it literally flew out of his hands, and into a tree. The stock was a little dinged, but other than that, it was fine. I'm not sure the 870 would do the same.
P.s The shell did have to much powder in it. after cleared the gun i checked out the shell. it had a bunch of unburnt powder in it, and the top had basically flowered.