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.
If a man neglects to enforce his rights, he cannot complain if, after a while, the law follows his example.
Without idiots, there would be no baseline for common sense.