The barrel in criogenically treated which should give you a little better shot pattern, the M2 will have less recoil b/c of the chevrons, the choke tubes are longer which will also help your shot pattern. The M2 is a much better gun and the barrels are not interchageable.
I know that you have the remington and it was free but in my opinion the Benelli is just such a better gun. I have friends that use that Benelli (guides) everyday and have never had a malfunction with the gun and those guys never clean them. You will have that Benelli forever.
I don't think that that your are going to be able to add a recoil tube to the M2. You can do that to the older models but not the newer ones. You might look at the youth model montefeltro and buy him the full size stock when he is ready for it. You really aren't going to be able to cut the M2 stock down either.
I would for sure look at the Cordoba. That is probably the best cross over gun that Benelli makes. It has a great feel and has very little recoil. It will have the same or less recoil than a gas gun.