I want a autoloader shottie to SBS to 14" for HD and general purpose zombie defense.  I can't find the difference between the M1 and M2 anywhere.  

Marketing bit fom Benelli's site:

Benelli engineers have also adapted the ComforTech™ recoil system for the M2. This revolutionary system reduces felt recoil by up to 48%, and cuts muzzle climb by 15%. When combined, this puts the shooter back on target up to 69% faster than with any other comparable semi-auto shotgun. The M2 is not only faster, but with the new ComforTech™ system, it’s arguably the most comfortable shotgun you will ever shoot.

To put more pellets on target the M2’s barrel and chokes are cryogenically treated, frozen to -300° F, which changes the bore surface at the molecular level making it more uniform and relieving stresses in the steel. The end result is a denser and more uniform pattern that puts up to 13.2% more pellets on target.

I'm sure others can bring the dirt to the table...

eta: I think the M2 tacticals have the reciever drilled and tapped.