If you want a SA/DA gun, then the Walther P99AS is the only horse in the race (and it is pretty damned nice anyway).
Between a Glock 19 and Walther PPQ, I chose the PPQ.
I think the argument in some ways boils down to this: The PPQ is the better gun, but the Glock has better availability for basically everything.
Why I think the PPQ is a better gun:
1. The trigger is the best of any of the "safe action" style triggers out there, and is good enough that it rivals or surpasses the single action triggers of some 1911's and many da/sa guns. It is the best trigger because:
A. It is smooth
B. It is the lightest
C. It has the shortest reset (and is thus the fastest)
D. It has a comfortable curve to help your finger maximize the other qualities
2. The gun's ergonomics are fantastic. The Glock ergo's are typically love it or hate it. I am firmly in the hate it camp. That is preference, but I've never seen (or frankly read of) a person who held a PPQ and said, "the ergonomics suck."
A. Interchangeable backstraps that make noticeable changes in the gun's fit
B. Frantastic purchase on the frame because of the HK style texture
C. ambidextrous everything
D. The magazine release takes a little getting used to (I was because I have a P22), but once you do, I believe it is faster, easier, and more conducive to keeping a consistent grip
E. The slide serrations are useful
3. The guns are now equally priced (or the PPQ is cheaper). I paid $520 out the door. I have started seeing PPQ's for $480ish online. Glock Gen 4 is right in that neighborhood.
HOWEVER
There is no denying that the Glock has more aftermarket support, availability, and generally at a reduced price.
1. It looks like it is going to be hard to get Walther spare parts, and Glock parts are easy to get. Smith and Wesson does Walther's importing, and it looks like they hoard a lot of the spare parts.
2. Magazines for the Walther are ridiculous. They are hard to find, and are expensive. If you are the kind of guy who buys 10 magazines for every pistol he owns, then you can delete my argument that the Walther is cheaper. There are some workarounds on the Walther mags, but even those are still expensive.
3. Holster availability is definitely pro Glock. Every holster manufacturer makes like 3 models for a Glock 19. Manufacturers are starting to really jump on the PPQ...but still, even best case scenario, Glock 19 will rule.
4. Sights, barrels, other stuff...Definitely Glock.
So, obviously, even with those negatives, I came down on the side of the PPQ (and I will also someday own a P99AS), but that is basically how it broke down in my mind.