Well, I'm not. First, the P239 isn't very little. I know, because I have one in .40S&W. I carry my Kahr K40, which is much smaller. Second, I prefer DAO over DA/SA (yes, that's a religious issue). Third, .357Sig ammo is significantly more expensive than .40S&W.
I'm not criticizing the P239. It's a fine, accurate gun. But there are other guns in similar calibers which are much more size efficient (e.g., Kahr K40, Glock 27, etc.).
So I think the folks at that gun store are blowing smoke.
Jared