Holy Cow!
Where do I begin. I don't know why I feel SO compelled to respond to some of the comments, but I just had to.
I'm getting read to purchase the Walther S&W 9mm, but I'm confused about the different models available so I came here.
When it comes to Single Action (SA) versus Double Action (DA) there are two ways to look at it.
Wait, before I head down that road, let me start off by saying that the QA and AS are both technically double-action only, not SA/DA like some people think.
A true Single action releases the hammer to strike the firing pin which strikes the primer which ignites the bullet. The "detent" on the QA only buys you half a pull, it still functions (technically speaking of course) like a DA pistol, because you're still raising the hammer to striking position as you pull - not simply release the hammer. Having said that, it feels very much like SA because of the light (roughly 3 lbs.) pull required to discharge the weapon.
Ok, shew....back to SA vs. DA. SA is more accurate than DA. Case closed. Nobody will EVER argue the opposite. However, in stressful situations a trigger that changes behavior can throw a person off and can be difficult to handle. Yes, the gun may go off sooner than expected because you're expecting 7 lbs. and 3/4 inch travel instead of 3 lbs. and 1/4 inch of travel.
Still SA is more accurate because squeezing the trigger doesn't disrupt aim as much. So, what do you do? Good question. There's a little known technique in the gun world called "practice" - you should look into, it's great. If you honestly expect to use a weapon with surgical precision the when the day comes when it's you or him - you better darn well be at the shooting range at least once or twice a week. I don't care if you have to go to the range every week for the next 20 years before your "big day" comes - that's what it takes. It's either that or shooting an innocent bystander or yourself instead of the assailant.
So, (Insert Your Name Here), buy the pistol with the SA (or in this case the "detent"), hit the range and hit it often - and pray that you never have to use your gun to end the life of another human being. But if you have to, make sure it's the life of someone trying to take yours, and not someone else, or worse - yourself.