Major is the powerfactor.
You can shoot minor, but no one wins shooting 9mm minor in USPSA, you just can't catch up to the .40 and .45 guys shooting minor.
If you plan on shooting competitively, you'll either have to handload 9mm or .38 super, or shoot factory .40 or .45.
At least, that's what I have taken from similar conversations. I'm no expert, as I don't shoot USPSA, or handload.