Persoanlly, for the Benelli, I would go with a factory mag extension. These can be had directly from Benelli (at retail) or bought from or ordered through a Benelli dealer.
The reason I would go with the factory mag (the ones that add more than 2 rounds) is that is has a step (wide circumferential groove) along the length of the tube that accomodates the barrel clamp (which is included with the mag). This step helps keep the clamp in place under recoil plus, IMO, they are of higher build quality even though they are just nothing more than tubes. The aftermarket mag extensions like the TacStars and Choates don't have a groove and as a result, the supplied clamps usually walk forward, particularly after shooting heavy loads.
