Stock.
The shotgun evolved without a pistol grip and with a stock for a multitude of reasons.
This is a variation on a basic law of nature: "If nobody uses it, there is a reason."
That said, go with what YOU are most comfortable shooting. If you really have practiced a LOT with a pistol grip and find it better than a stock, shoot a pistol grip. But most people I have seen with PG shotguns (and thats not very many) get serious wrist pain after a couple of rounds, and either go back to stock shotguns or (stupidly) keep the PG and just don't practice.
Whichever you choose, you GOTTA practice.
Mike
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"A human being should be able to change a diaper, plan an invasion, butcher a hog, conn a ship, design a building, write a sonnet, balance accounts, build a wall, set a bone, comfort the dying, take orders, give orders, cooperate, act alone, solve equations, analyze a new problem, pitch manure, program a computer, cook a tasty meal, fight efficiently, die gallantly. Specialization is for insects." -Robert Heinlein