I'll make the assumption that you are talking about full stocks with a pistol grip.
both the Choate and Speedfeed stocks seem to work well and are sturdy enough to take the ruff handling of prolonged service.
both companies offer models that allow you to store extra ammo in the stock. in this the speedfeed is probably the better bet. while choate has the advantage of being virtualy indestuctible.
While I have used the Tac star side saddle in the past, on a 20" Mossberg 590 w/ ghost rings
and liked it on the heavier gun.
I have taken the minimilist approach with my 18" 870. When I think of the 870, I think, Light, fast, hit hard, and just cannot bring myself to hang something on the side that throws of the balance of the weapon and thus detracts from its wonderful, quick, point and shoot capabilities.
I will leave you now with this final thought...
The guy carrying the riot gun, is the same guy that would have been carrying a battle axe, had he been born in the tenth century. The mindset is the same.