I also had a chance to get a S40 used at a very good price but went with the A1 and am well pleased that I spent the extra 300 bucks.
Here is a snippet from Wikipedia on the differences
http://en.wikipedia.org/wiki/Steyr_M
Variants
Steyr M9-A1 with magazine and tactical light.
The compact Steyr S9-A1.In 2004, an improved version of the pistol replaced the Steyr M in production. The new Steyr M-A1 and S-A1 pistols received several improvements. The grip of the pistol has been redesigned with some textured surfaces as was the magazine well (uses the same magazines), ergonomics have been slightly altered to improve grip, the manual safety button is now optional (not in models sold in the United States, all US imports lack the manual safety) and the lower forward portion of the frame now consists of a STANAG 2324 Picatinny rail for mounting accessories.
In 2010, Steyr Mannlicher US began reimporting Steyr M-A1 and S-A1 pistols.[5] The newly imported Steyr M and S have a revised slide imprinted with the Steyr Arms logo.[citation needed] The newly imported 2010 models have a roll pin in the slide improving trigger pull and a modified extractor for easier ejecting of casings.[citation needed] Due to liability reasons, all Steyr M pistols imported to USA are not available with the manual safety.
You can check the 3 digit date code on the slide to make sure it was manufactured on or after 2010 which should read *WO or *WW with the * being the month code.
Hope this helps.
