The P99 QA does have a slightly shorter trigger pull than the P99's single action pull. However, the QA has a slightly higher pull weight.
If I were you, I'd reconsider the double action/single action model. The trigger pulls of the P99 and P99 QA are very similar. However, I feel that as a carry weapon, the P99 DA/SA is much safer. A decocked P99 will fire in DA mode, but a decocked P99 QA will not. So to keep the QA ready, you have to have it chambered and half-cocked. Also, it is just as easy to cock a P99 as it is a P99 QA.
The P99 has second-strike capability should a round ever misfire. Although the P99 QA is a DAO pistol, I am not sure if it has second-strike capability since it fires from a half-cocked position. I'm sure somebody on this board can provide this useful information.
If the QA it not able to restrike a misfired round, I would think that reason is enough to go for a P99 DA/SA instead.