I'm an average (at best) 3gunner, but I do have the good fortune of shooting with guys like Rob Romero and Erik Lund since both are local. Rob has been a team shooter for both Benelli and Remington. Currently, not having any shotgun sponsorship, he's shooting a Benelli. According to him, the barrel lock up on the Remington Versamax does not allow for consistent slug accuracy. To me, an inertia gun wins over a gas gun for long term reliability's sake. They are weight sensitive, but test your gear, and you'll be fine. My M1's run bulk target loads 100%. As far as pistol grip and light placement, that's 100% personal preference.
Pat Kelly and SLG's wife wife have more 3gun experience than I do. Maybe they'll chime in.
I know 3gun isn't your ultimate goal, but is one of the few proving grounds for shotguns now that most things on the tactical side are centered around the carbine.