My deal is, I don't want to get a rifle/carbine in a caliber I don't already own in a handgun. I don't want to buy a handgun to go with the new rifle and have to retool and load for another caliber. This limits me somewhat, but still gives me choices.
Currently 9mm, 38/357, 40S&W, and 45ACP.
The only cartridge that gets me a large gain in usability is the 357mag. It also gives me versatility because I can use the 38 special in the same gun.