What is the difference between...?

The name. They are the exact same gun. It's just one has a "M" for the military designation.

The military version of the Berretta pistol is just a 92. Its not an FS model, just a plain 92. The M9 is the military version of the stripped down 92. They are basically the same physically, except for the serial number sequence is different on the M9's. The M9 being just a model 92 is just the bare stock model. The 92F, 92FS, etc are just different models.

There is a civilian version of the M9 available to the general public, in different colors. Just as the Colt 1911 series pistols are available to the general public in limited quantities in different colors.

Th 9mm cartridge is a bit under powered for self defense, but is more readily available, less of a recoil, and allows the magazine to hold more rounds (15 round magazines). The Colt 1911 .45acp is still a great pistol and a great self defense round, however the .45acp rounds are harder to get in combat zones, and have a higher recoil, and less magazine capacity (7 rounds typically).

Always details! Remember how the 223 mutated so much that modern military versions might or might not be a good idea in a Mini-14? And the same thing had already happened to the 308! It seems to not happen in handguns as much, but that dog has left a pattern of bites which might be expected to continue in some form. I guess we'll see.