PBR: First you need to decide on what you want:
-Revolver, Pistol, Polymer Pistol etc
-Then you decide on caliber...some glock calibers are glock specific. They are awesome calibers but they are hard to find and expensive.
-9mm is right for me because I get boxes of 50 for $9.99 from walmart, thats cheaper than most anyone on here will find ammo.
-9mm is easy to shoot and also easy to hand load.
-I can successfully double tap on a body sized mass and get great groups with 9mm, from about 15 yards.
In the end its your choice, but I will sway you towards 9mm because its my favorite for practicality, reliability and defense.