I would keep the Mossberg. If you need a barrel, chokes, after market stocks you name it, you can find it for a mossberg. Good luck with the rock island.
Right now you cannot predict what you are going to do with a shotgun. I never thought I would shoot skeet, have a slug gun complete with a scope or turkey hunt.
At 21 all I was interested in was quail and rabbits. I had a sweet 16 browning that I thought was the only shotgun I would ever need. Right now shotguns are multiplying like rabbits around here.