If you're looking to go small, here are my thoughts, from the perspective of someone who primarily runs handguns, sets people up with them, and trains with them.
-9mm. Less recoil, less expensive, greater capacity, and a great spread of options in ammunition.
-Glock 9mm or S&W M&P in 9mm
-HIGH QUALITY HOLSTER. This will probably make the biggest difference among acceptable pistols. I carry my full size M&P with no problems in a NTAC holster.
Then buy a few extra magazines and ammo. I've not set up anyone with a Glock or M&P that hasn't been happy after getting proper training. I typically run a M&P Full Size 9mm when I'm running personally owned gear, but if I were going for upmost concealability I'd run the Shield. Most people I set up go to the store with me and handle the Glock 19, 26, and 17 alongside of the M&P 9mm in Full size, Compact, and SHIELD when it's available.
Most of those people pick the M&P, though I did have one oddball start with the M&P, go to Glocks, and end up wishing he'd stuck with the M&P though he's happy with his Glocks. Regardless, once you pick your pistol get some training with it. I like Combat Focus Shooting's program, and would recommend it over most of the training I've recieved in the military, and all of the training I've recieved from the military concerning handguns.
If you're looking to go "light", I think the Shield in 9mm will be your best bet. It's on my short list. The Walter PPQ seems to have a good following as well, but I have no experience there. Just remember to get something that you're actually going to be willing to carry, instead of what your buddies(including me) think is cool.
Have fun!