If you do look at keeping the 500, I would recommend you call Mossberg directly if looking for barrels. They can tell you every barrel option available.
In fact I just ordered some more spare parts including another barrel with rifle sights just last week. 1-800-363-3555
It was about $40 cheaper than anywhere else I found.
I agree with Blindstitch. For a pump shotgun that takes 3.5" shells, Mossberg 835 is hard to beat. They can also usually be found frequently in the used gun rack at places like Cabela's.
I used to have several different shotguns, but it is now the only shotgun I own. It covers all of my bases, from hunting to home defense.
However I also agree with 75218ron, you also likely don't need 3.5" shells. The only reason I went with the 835 is occasional waterfowl hunting with my overbore barrel and goose loads, or the option for turkey loads should I ever decide to take up turkey hunting. And I didn't really need 3.5" for that.
What's your intended goal? Hunting? If so, what critters? Home defense? Clay busting or shooting competition?