i have taken apart my 590, 500, 870, and winchester assault shotguns. from a mechanical perspective, i have more trust in the 590- there is just less that can go wrong. the remington just isnt made out of big, strong parts. too many little sheet metal parts in it for my tastes.
from shooting experience, the 590 slicks up nice with some use. like i said, run some rounds downrange and it will slick up nice. i have a model 500 20 gauge that i havent cleaned in well over 5000 rounds. i have shot the hell out of it, and i aint going to clean it because i want to see just how much it takes to make it have problems. right now, the interior of the gun is filthy. try as i might, it just keeps on shooting. you cant beat a mossberg action for strength and reliablity.
the truth is, either will do fine for you. in the end personal preferance is going to be a large part of your decision.