I did own the Walther p99 QA. three different triggers with the P99. Make sure you like the trigger if you go with the walther. Walther has great ergonomics, great reliability, but trigger was hard to get use to, but with a few hundred rounds it was very accurate.
The P99QA is generally regarded as the least-desirable P99 variant due to its trigger. The QA trigger is very Glock-like, but heavier at ~8.5 lbs.
The PPQ (called P99Q or P99RAD in the European market) has replaced the P99QA, and has a much lighter and smoother (~5.5 lbs.) Glock-like trigger with an incredibly short reset. Most folks who have this gun love it.
The P99AS and first-gen P99s have the superb AS (Anti-Stress) DA/SA trigger. This is the variant you want, IMHO. This a very easy trigger system to use, since it works pretty much just like most DA/SA pistols out there, but has a remarkablly smooth and crisp trigger with the same incredibly short reset. The single-action trigger pull is ~4.5 lbs, which makes it a fantastic range pistol, and the DA/SA action makes it a safer alternative for defensive use or carry, IMHO.