honestly the main 500/590 feature i miss on my 870s is the fact that the lifter stays out of the way in the up position, allowing you to load shells into an unobstructed mag faster/easier. my girlfriend, being a lefty, has trouble with the 870 slide release and she would probably do better with a Mossberg. the safety is not such a big deal: yes, a tang safety is more ambidextrous and convenient, however its useless if you install a pistol grip.
i like the fact that the magazine tube can simply unscrew from the Mossberg receiver, as opposed to the brazed-in magazine tube of the 870.
the 870 ejector is riveted, but ive never broken one. more likely, you may break the ejector spring - but the gun will still eject without it. the 20 gauges dont even have one to begin with.
but most of all i prefer the fact that i can go to the EE, or pretty much any gun store, and find anything i need for the 870. it has the largest aftermarket of any shotgun in the world, and the largest userbase. new, used, take-off and aftermarket parts are everywhere, and you can turn an 870 receiver into anything. not so much with a 590.
either way you cant really go wrong in buying an 870 or 500/590 model (and ive had very good luck with my Grizzly clones, albeit if it werent for the fact that they come in barrel lengths unavailable in Remington 870s then i would have purchased used Wingmasters/870Ps instead). IMO just stay away from the 'off brands' like Hatsans, Tomahawks, etc.