Re the P99QA- "As I understand it" the difference between it and the standard P99 are trigger pull, travel of the trigger, and whether the gun can be completely decocked. The P99QA has a rather short travel for the trigger, but the trigger pull is roughly 7.5 lbs. The standard model has, in my opinion, a long travel for its trigger, but a lighter trigger pull which is closer to 5.5 lbs. (There is a graph on Walther's homepage to illustrate the concept.)
http://www.waltheramerica.com/p99_specs.htm
Also, the P99QA can be completely decocked by using the decocker, whereas the standard P99 reverts from what is essentially a single action with a chambered round to double action. In contrast, you can squeeze the trigger of a decocked P99QA and it will not fire until you re-cock it by partially pulling back the slide as though you were checking to see if you had a round in the pipe. You'll hear a click and you're ready to go when you release the slide.
-I don't know if this will enter into it for you, but some pistol competitions will require that you decock the standard P99 so you'd be firing it uncocked for the first shot (with the long travel depicted in the graph). Of course, the following shots would have a shorter travel, but many find it annoying to have the first shot require a different trigger pull.
-I like my trigger pull and travel to be identical for every shot, so I think the P99QA has a better action, but I also think the lighter trigger pull of the standard P99 is desirable because you can feel the 2 lbs. difference. -However, it's all up to your personal preference.