With the 9mm you have several options for uppers ie: RRA, Colt, Olympic, or even (God help me) ASSA. The 45acp is limitted to Olympic Arms. The 9s hav high (or normal) capacity mags of 30, 32, 0r 34 (with the Sterling mag) rounds. The 45 is limitted to 16 rounds with a modified Uzi mag. It seems like there have been just a whole lot more options with the 9.
Having said that, Cav Arms has a lower that takes Grease Gun mags comming out. When that happens (if it has not already done so) the 45 might take off like a rocket.
The final factor is price. The parts are essentially the same, but ammo cost is much higher for the 45. Even reloads are pricey. I can get 9mm down to 5.5 to 6 cents a round with lead reloads. I'm looking at 10-12 cents a round for the 45. At 500 tfs (trigger finger speed) that adds up.
Bottom line, get what makes you happy. Or as I have seen so often here...GET BOTH
ka