If your going with a pump action shotgun, you have no restrictions concerning pistol grip stocks or folding stocks or mag extensions. The rules change when you have a semi-auto shotgun with a pistol grip or folding stock, then you may not put the extension on. But to answer your question, you may increase the guns firepower and put a cool pistol grip stock on it! The Mossberg Persuader is an 8 shot variant of the classic M500. It has a mean looking heat guard and comes with a pistol grip. Remington Home Defense 870 shotguns come with the standard stock (no pistol grip) and 4 round magazine. The suck thing about the new HD models is that they have dimples inside of the mag tube that prevent the owner from installing an extension (some say you can file em' or drill em', I'm not that abusive on my guns) My personal favorite, the Mossberg 590A1, with nine shot mag and ghost ring sites. It is a military contract shotgun, it passed some harsh MIL-SPEC testing with out a hitch. It has a hardened parkerized-rust resistant finish and 20 inch barrel. Remington does manufacture an extended capacity 870 shotgun called the Police Model. Civilians can legally purchase these, there are no laws restricting them. The gun has an 18 inch barrel with 7 shot magazine. It is like the HD but with wood stocks and mag extension.
Mossberg 500 persuader $250-300
Mossberg 590A1 $350-400
Remington 870 HD $200-280
Remington 870 Police Model $380-450
I live in the Southern California area, so these prices may or may not reflect your areas prices. Good luck!