I've been looking into getting an HD shotgun and pretty much have it narrowed down between the 870 Express and Mossberg 500. Is the only negative with the 870 the non-interchangeable barrel? That wouldn't be a bid deal to me as I already have a "hunting" shotgun.
A regular 870 express is good to go. It the tactical models that you cant change the barrels on. I would take a 870 over a 500. Both are good guns it just personal preference and it always seems that the 500 tends go rattle more.
I ended up buying parts here and there for my Wingmaster much like I did for my 10/22. When it's at home for the most part I put on the mag extension and the 18.5" barrel. When hunting season comes around, I remove the extension and put on the appropriate barrel for bird or deer. Keeping it all in a hard case makes for a tidy package. I've had it for many years and it's served me well.