As Michael does, the gun I carry is based upon my personal threat assessment for any given day. My normal every day carry gun is a 9mm Kel-Tec P11. This is a very easily concealable weapon whether it's carried in an inside the waistband holster, a pants pocket, or a fanny pack. On the days when I am doing something or going somewhere outside my ordinary threat level, I carry the Kel-Tec in my pocket and my S&W M&P .40 in a strong-side, inside the waistband holster. After all, to quote Clint Smith, "One is none, two is one."
As many posters have correctly asserted, the perfect carry gun is the one that you'll carry every day. "Leftit" guns (you know...leftit in the car...leftit in the house, etc) are worse than useless. I can't think of a more sickening feeling in the pit of one's stomach than, in a serious social encounter, to reach for the gun you normally carry only to find nothing there but waistband.
And if you choose to carry, PLEASE be certain you have what it takes to make the damn thing go "boom" if the siutation calls for it!! As much as I do NOT want to be killed by some hulking cretin, I even more do NOT want to be killed by some hulking cretin using my own damn gun!!