The SAS stands for Sig Anti-Snag. They do the "melt job" on the slide, and dehorn other things like the sights and hammer. It is a Double Action/Single Action pistol, so it's going to fire DA if the hammer is down, and SA if it's cocked.
The DAK is Double Action Kellerman. It's a double action only with a few features. It has a light pull weight when a round is chambered because it's at quarter cock kind of. When you pull the trigger, one of three things can happen...if you have a light strike on the primer the hammer will drop and stay there, because the slide didn't move back to quarter cock it. You will have a slightly heavier trigger pull for a second strike. If the round fires, which it should, you can let the trigger out halfway, it will reset and you get another slightly heavier pull, with a shorter stroke. You can also let it out all the way, and it will reset to the normal trigger pull. Sounds a little complex, but it's really simple, very smooth, and I love it.