I mostly agree with dannybot's post except that I would go for the .40 and then buy a second 9mm barrel and 9mm mags. You can put the 9mm barell in the .40 gun, but you can not put the .40 barrel in the 9mm gun.

For just a little more money you have the option of two guns. I personally prefer the .40 for SD, but then having the ability to practice with the cheaper 9mm rounds will pay for extra barrel and mags pretty quick.

You should be able to get the extra barell and two mags for around $100.