The metal parts and thicker barrel are not worth a dime of price difference in my opinion.
The 590 and 590A1 lines have some variations of finish, barrel length, and sight options that you don't find on a regular 500. Unless you really need one of these it's not worth it.
Mossberg made the 500 for many, many decades before the 590's, there's really not anything wrong with the design as-is. The mag tube nut is more convenient, I will admit that.
edit: I've shot a 500, own a 590, have fondled a 590A1 but not shot it.