The M2 did in fact replace the M1 and the biggest differences are external design and maybe some features that weren't offered on the various M1 models. The SBE has always been the 3.5" auto for Benelli and it's essentially the only difference between the two. All Benelli autos are still inertia operated, though the breakdown for the SBE is slightly different than the M1 or M2. If you "need" a 3.5" shell then you've answered your own question, but I find that pretty much nobody ever needs a 3.5". I have an M1 and I also have a Browning Gold 3.5". I have never even purchased 3.5" shells for the Browning. Both guns have custom chokes for both buckshot and heavy shot (which I use the same load for turkey and waterfowl) and both pattern so well there is nothing left to be desired. Keep in mind that for many years 2 3/4" shells did everything people needed to do so a 3" shell is hard to beat. I wouldn't spend the extra money on the SBE.