Personally I would go with the Benelli M2, over a Benelli pump. I have an M2 with a 26 inch barrel which I use for pigeons and geese.
With a pump, recoil will be a lot more noticeable with the same cartridges and it's a bit of a chore to manually work the pump when birds are coming in thick and fast when an auto will do the work for you, allowing you to solely concentrate on the shot.
One of my friends bought a new Winchester pump to go goose shooting with, with 50 gram plus cartridges, it was not at all pleasant to shoot, recoil felt about double what was experienced with the M2 with the same loads!