I'm a little bit different in that I use a pump. Remington 870. The reasoning for me is simple. I have used the same shotgun since I was 12 years old. I have hunted with it, and with a short barrel and extension it usually sits in the top of the closet as my main HD gun. Loyalty (and therefore familiarity) have got to be worth something.
But Spd522 is right on. The guys shooting the semis usually win the shotgun stages. The last two 3gun matches I have been too, the top shotgunners all used semis. I was the fastest pump, and came in always just behind the top two guys (one Benelli, one 1100). There wasn't a whole lot of difference in split times, but it was there.
I'm shooting my same pump in the WC3gun in Las Vegas in Sept. At least there they list if you are shooting a pump or semi.
Pistol grip or regular stock all depends on what feels good for you. For me once again it comes down to familiarity and I use a standard stock.
Some matches require large round counts, make sure you can reload fast. Most of the hard core will have side saddles mounted on their guns.
Mag extensions are a must have.
Sights are once again a personal thing. I'm not much of a slug shooter, (not a whole lot of slug hunting around Utah!
)and I just use a regular bead sight. At just about any regular shotgunning distance I don't aim, I just point. Kind of like shooting clay pigeons.