Agreed we have a lot of firearm enthusiasts on here! Also military and techs, it seems.
I would agree with both the 870 and 500 being excellent choices, and it would come down to feel or the better deal.
As an aside, have you considered the Kel-Tec KSG? 14+1 of bullpup downwards-ejecting Picatinney-equipped pump 2¾" 12ga slugs or buck ain't bad. The price is good, it's ambidextrous, shoots 3" shells (albeit with less capacity), slugs, even less-than-lethal if that's what you're into. With the center of gravity near the grip, it has less muzzle flip than a conventional pump because the recoil goes straight back. A downside would be you're not used to reloading tubes on a bullpup with your off hand, but with two 7-round tubes the springs can be lighter and you can put buck in one and slugs in the other if you want.
18½" barrel to keep from paying the Feds more taxes, but the bullpup design means the whole thing is only 26" long. Kel-Tec has been a pretty innovative company, with their little .32 and .380 (now copied by most everyone), the 19oz (loaded!) 31-shot .22 Mag pistol and their .308 threaded muzzle bullpup rifle.
PS Zo, I know I didn't have anything concrete on why the Mossie or Remmie is better; mainly I didn't want to pass up another gun thread! [guiltily hangs head]