The A1 is the 92fs with some upgrades.
The most notable issue with the FS was the locking block failures, and the A1 has addressed this with a recoil buffer.
The A1 also offers a removable front site where the FS was part of the slide, so there was no option to change sights.
Obviously the rail is another feature, although it didn't matter to me.
In the end I chose the A1, even though I was really drawn to the stainless FS. The A1 is pretty much the same pistol with a few upgrades, and is the only way to go IMHO.
I am very happy with my purchase, and the fit and finish is far above anything else in this price category. I have owned all of them and I havn't been as pleased as I am with the 92A1.