Seems there is now a slight lean towards 9mm, which would be my suggestion too yet surprises me, given that here are Americans (I reckon they are?) I would have figured a clear vote for the .45.
As for the 357 SIG, I learned that it's not easy to tame for the common shooter, so maybe not the best choice for a weapon that would be issued accross the armed forces from assault teams to echelon staff... Actually I didn't add it because though adopted by certain federal agencies, I reckon it's far from being as popular as the .40 S&W.
In general, of course the sidearm is far from being the most important issue in modern weaponsry, but I guess it's still an interesting topic out of two reasons:
> being a weapon of last resort makes the sidearm choice a rather emotional issue apart from the fact that it is not too important in actual warfare
> in the US, handguns may still have some symbolic value reaching back to the "cowboy" days, thus being connected w/ individualism and activism...
well, enough inputs for now...