+1 on the 10B. I've got quite a bit of trigger time on two of them back in the '80s. I even shot a round of sporting clays with one, and it didn't do half bad on the close stuff. Somewhere there' a picture of me dual-weilding both 10B's at arms length at 10 pepper poppers, huge muzzle flashes going...
That said, my 18" 870 is tiny and effective, and the price difference was significant (only $140.0 for an 18" Police 870 with a factory extension) so..