I'll take the wishy-washy path and say: Get what you want. Just start shopping, and something will begin to resonate in the depths of your soul (if something hasn't already!) There will be "that one" that you keep coming back to look at, that you compare all the others to.
If you don't reload, I would say get another 9mm. Supply/demand is crazy right now, but historically, 9mm is always cheaper than .45 ACP. And, since you already own some 9mm guns, you likely have some ammo stashed away. Also, a 5" 9mm 1911 is a cupcake to shoot. I own 1911's in both 9mm and .45 ACP; the 9mm is fun for everyone to shoot.
The best arguments in favor of a .45 ACP are (1) You don't own one. Everybody should. 'nuff said. (2) You can craft a wide variety of ammo if you reload. You can spend the better part of a day punching holes with a 200gr SWC over 4.0gr of Clays. It's very therapeutic. (3) When you get the itch to "shoot a bigger gun" the .45 ACP with 230gr up to 255gr bullets is most satisfying. (4) Saint John Moses Browning made it to be chambered in .45, and you can't improve on perfection (see reason #1, supra).
Unfortunately, I can't really endorse the .38 Super. I know others will flame me for such blasphemy, but my buddy owned two 1911's in .38 Super. I shot them both a fair amount. They don't really feel significantly different than a full 9mm load when you're shooting. Of course, if you want "cool factor," people will slobber all over themselves, complimenting you and your .38 Super. But, I'd only recommend it if you own some sort of a progressive press. Given the low recoil of the .38 Super, you'll shoot as much ammo as you would a 9mm. Single-staging handgun ammo is brutal. Paying retail for .38 Super ammo (if you can find it) is more brutal. But, like I said, do it only for the cool factor, not the shooting factor.
Good luck in your decision! Be sure to post pics of the new acquisition--because when somebody gets a new gun, it's like we ALL get a new gun!
Carry on!
