The Sigmas can be purchased new for $299, so I would not look upon a used price of $250 to be a good one. There was a $50 rebate for a new purchase as well, but I'm not sure if it went past the end of Dec.. Check the S&W website, as $250 for a new one is not a bad price at all. Personally though, I feel that a used Sig or Glock, or even one of the Walther P1's left over from the German army makes a better choice than a Sigma, mostly because of the much better trigger pulls.
The P22 is usually about $299 new, and can sometimes be found on sale for $250. We have one, only because my wife likes to shoot it. Overall though, a Ruger or Browning makes a better .22 auto..
A 9mm would make a better home defense/nightstand gun that would a P3AT, but then again, a .380 is better than nothing. I would say get a larger handgun for range/practice/plinking/carry/home defense use, and once you have enough money, get a P3AT for back-up or deep cover duties. IMO the P3AT is meant to be more of a concealed-carry weapon and not as a home defense gun, especially since one doesn't need to worry about overall size in an HD gun...