The thing to consider now is if you want the new quick detach sytem they have come out with or if you want to stick with the tried and true mossberg design. In my experience its best to wait for them to work the bugs out but I havent had a chance to get my hands on the new system yet so I cant give you any personal input on it
Been a lot on these forums latley on the 870 / 500 matchup. Both are solid and proven shot guns and both have their pro's and con's. If you got either one you would have made a good investment. Good luck.