When rumblings started about Glock doing injection molding of frames here in the states (about four to five years ago), I lobbied Glock to start development of a single-stack/slim-profile version of their G-26 Subcompact (basically something akin to the relationship between the G-30 and G-36, but in the smaller-framed 9mm line-up). I was told there was no market...
Then, two years later, I had the pleasure of walking Glock's US VP (head of Glock No. America) over to the S&W booth at the NRA Annual Meeting in St Louis, and showing him the PPS. Told him that apparently Walther had tapped my phone lines, and that he (Glock's VP) made a very bad call, especially in light of all the CCW legislation that has been passing all of the US in the last 10 years or so.
As it turned out, someone from Ruger apparently overheard that conversation, because the LCP came out a year and half after that...
To be honest, I love Walthers, but I was not overly impressed with the ergonomics and appearance of the PPS. It's size is excellent, but the shape just didn't fit me, both in my hand and by eye. (Definitely doesn't mean it wouldn't fit you. If you get a chance to test one at the range, it would be best, but just handling it in the gunshop will tell you if it's OK or not.)
I went the route of the Ruger LCP with Hornady Critical Defense ammo. It's the size of a 25ACP (O.K., at least as small as a 32ACP), and it has almost the wallop of a 9mm (light-bullet, non-+P for sure) with the Hornady CD ammo. (When I say that I'm merely looking at diameter and penetration, not energy.)
I find the LCP to be not too bad to control shooting, fairly accurate (considering the near-absence of sights), and VERY concealable. I work in LE, and no one at work noticed I had it in my pocket until I showed it to them, and I didn't show it to anyone there until I'd been carrying it for three months...
I've read other's here recommending obviously bigger and heavier pistols. I take it from your question (and I might be wrong), that concealment and being inconspicuous are bigger factors than firepower. Therefore, I recommend you look at the LCP.