As for the 870 or mossberg...which one is more reliable? Plan on using as HD.
Personally I'm a 870 kind of guy. With that disclaimer out of the way there is no difference in reliability. Both are uber reliable. The only thing you'd have to keep an eye on is the 870 Express models sometimes have rough chambers that cause cheap practice ammo to stick (easily fixed with a little steel wool). I polish the chamber with steel wool before I even send a round down range so I've never had an issue with any of them. 500s and 590s are great too.