Be sure that any 290 that you buy is a P290RS. This means that the trigger can be pulled multiple times on the same round. (a big help if you have some chintzy ammo) Most P290s are marked as RS on the slide, but some of the early ones were factory modified and not marked. The treason for the modification was to correct a problem. I don't remember what that problem was. The P290RS is a good carry gun.
The triggers on the 250's are nice. My wife gave me a 320 for Christmas, I liked it so much I now have two. I plan on checking for a 22 conversion kit for it soon.
I hope Sig or the aftermarket comes up with a 22 LR conversion or Xchange kit for the P320. Not sure if this is possible technically, but I think it'd be a hit if done right.