I think it's one of those amount of expected use things. If you hunt all the time, or shoot trap weekly or in some other way get the full use out of it, then the intangables like nice finish, better parts etc, become important.
If you just want something to carry in your truck and blast clay birds tossed by your friends on a 4 wheeling trip in a casual way, then the Express is ideal.
The Express is a great gun for the money, but if you take that out of it, it's just an OK shotgun. An 870 Wingmaster is a GREAT gun and the standard by which all pumps are measured, whether you factor in price or not. As was mentioned above, it's a gun your grandson will still be shooting 50 years from now.
Edit: If the $500 means you don't buy it at all, buy the Express. It's the best $275 you'll spend this year. But if $500 more won't break the bank, the Wingmaster is a very nice shotgun and still not huge money.