I am going to make the assumption that you will be using a glock. With that said if you buy a 9mm you can only shoot 9mm. If you buy a 40 or 357sig you can shoot the other with just a barrel swap. You can also shoot 9mm with a conversion barrel and the right mags. I would recommend you buy a 40 with the two extra barrels and some 9mm mags. I only say the 40 because if you want to sell the gun in the future it is a lot easier to sell a 40 than a 357sig.
When dealing with Democrats, let us remember we are
not dealing with creatures of logic. We are dealing with
creatures of emotion, creatures bristling with prejudices
and motivated by pride and vanity.