In addition to John's concise answer, a pistol grip on a full-length buttstock allows for better recoil control while firing from the underarm assault position. This position, used in close quarters or houseclearing, favors the pistol grip stock.
The pistol-gripped full stock is said to be less retainable in weapon retention situations. My longarm classes have not really addressed this distinction -- since the proper thing to do, unless you are well trained in longarm retention and disarming, is to let the goblin have your gun, draw your handgun, and let him have some of that too.
I'd say go with the conventional stock and 7 round magazine tube. You can still use a conventional stock in the underarm assault position and it is more retainable. The additional capacity is much more valuable, IMHO, than a pistol grip stock. Although I will admit the pistol grip stock does look devilishly kewl.
By the way, I had the same decision to make back in November of 1998, right before NICS kicked in. CDNN was closing out the last of the HK-stamped Benelli M1 Super 90 Tactical shotguns, for good prices -- you could get: (1) standard stock, ghost ring sights, three choke tubes, factory 7 shot magazine for $689; or (2) pistol grip stock, ghost ring sights, three choke tubes, factory 5 shot magazine for $699. An additional ten bucks would ship either gun to your favorite FFL in the lower 48. I chose #1 for the reasons above.
HTH,
Justin
------------------
Justin T. Huang, Esq.
late of Kennett Square, Pennsylvania