I haven't handled the Stevens shotgun, but I own an HP9 with 18.5" barrel. The bead on mine is slightly off centre (almost unnoticeable). The HP9 will definately have a much wider selection of aftermarket goodies, as pretty much anything that fits an 870 will work. Stocks, side saddles, foreends, mag followers, etc.
One thing to note about the HP9 is that 870 barrels will not fit on the 18.5" model - they will on the others, but the 18.5" has a non-standard barrel lug location. If you're happy with the cylinder bore barrel and the length, that's no problem. But if you ever wanted to change barrels or use chokes, you may be in trouble - I don't think the HP9's barrel is thick enough at the muzzle to be threaded for chokes.