Hmm lot,s of posts, but none of them shoot 3-gun or IPSC shotgun competition. I don't know very much but I do have a friend who shot a IPSC shotgun match once, so I will try to help out here.
An M1 or M2 is just fine, an M1 with a good recoil pad like a Kick-Eeze, or Limb-saver is THE same as an M2 with the comfort tech stock. The "preference for the M1" comes form the fact you can get a used M1 cheaper than a New M2, Both are just fine. I know a guy who has over 75,000 rounds through an M1 and it works just fine. Don't be afraid of a used one as hardly anyone will ever shoot that much. Just make sure no one has modified it so you can start from a clean slate.
Pistol grips are bad! The really mess up your swing in both directions and are hard to get to work on most Benellis. Only two of the top 10 action shooters use them and they just never win the shotgun stages.
Ghost ring Sights are bad. The block too much vision for rapid target acquisition A nice big fiber optic front with an express type rear sight or flip up open sight in the rear is what is winning.
Barrels between 21" and 24" are great.
M4s are way over priced and hard to modify for shotgun competition, save your money.
Which ever way you go, try to get a Field gun as opposed to a "tactical gun" as it will come with all sorts of shims and plated to fit the gun to you....THE most important thing in shotgunning whether it be action shooting or flying clay shooting.
Hope that helps. KurtM
You sure know a lot for knowing a guy guy who shot an IPSC shotgun match once. Just wanted that posted again in case some one missed it.