It appears you've really answered your own question....I noted you want the 3.5" option but why, when you use 2.75" or 3" already and are satisfied.
Save the $ and go with the 3" version.....(I am not a 'fan' of 3.5" anyhow!!!) .....in any make of gun! Also, I'm not sure why, but it appears that there are alot more problems out of the box for the SBE than the other Benellis - at least noted on the gun forums. Incidently, I have never seen a problem with a Montefeltro! I'll bet that could fir you better than an M2....give it a heft.