This is a tough one as trying to choose between a G21 .45 ACP and a G34 9mm is the issue. By far I prefer I G21 for carrying in the mountains, high desert, etc, and can carry it in town as well, but that takes a bit more effort. It can also be used as a competition gun very effectively, and I did for years in the 90s. However I don't really compete anymore and the days of the .45 in USPSA I would say are long gone.
The G34 is stupidly easy to shoot well and I very much enjoy shooting it. It also works quite well (for me) as a CCW piece when carried IWB. It is probably the easiest centerfire gun I own when it comes to getting fast and accurate hits on target. I have a couple that are more accurate, but the combo of speed and accuracy make it a standout. So with that in mind, I will make it the winner (for today).
I still am not sure that I like it better than the Glock 21 though. The G21 is simply a stellar handgun that is really a sleeper when it comes to doing so many things well. If I had the funds I would be picking up a Gen 5 and hunting with it. It could easily be the one .45 that does it all.
But for now, my old boringly reliable Gen 3 G34: