That marinecoat 590 A1 will probably outlive cockroaches and Twinkies, both!
The 590 is a beefed up 500. I think someone pointed out the main differences already. keeping the reliable design of the 500, the 590 A1's have a metal safety button, metal trigger guard/housing, heavy-walled barrel, and the receiver is a little thicker. I find the 20" 8+1 just too long/heavy for my likes, but for a high cap truly military pump-action shotgun I don't see how it could be improved. I'm fond of the shorter arm (sans bayonet lug though I love it on the 590 SP) with the SpeedFeed +4 stock (you see I'm a fan of) and prefer the Vang Comp +1 Mag Extension.
Sights? Mossberg's Ghost Ring Sights are fine except for one thing: they are Ghost Ring Sights on their shotgun. Also indestructible, they are SO cool looking but they belong on Enfield battle rifles, not shotguns. Unless you are shooting slow, aimed, longer distance shots only forget them. They are slower to aquire shotgun-range targets and block too much of your field of view as you might judge from the picture below. After initial testing I ordered the preferrable (if you are really a "shotgunner") bead front sighted barrel the next day, and currently sport their three-dot sight version.