10mm
For what it's worth, I have zero experience with .45 GAP but I've been carrying a full size Glock 20 for several years.
Couple things;
10mm ammo is expensive and sometimes hard to find. I use Cor-Bon ammo exclusively for hunting and usually carry it for personal defense and I've paid as much as $50 for a box. If you reload it might be less of an issue for you, but I don't just because of the time and patience commitment required.
10mm is LOUD and sharp and beats up your hand. I always draw a crowd at the local range because the G20 looks so nondescript it easily passes as any other full size, but when you fire it, it sounds like artillery compared to a 9 or a .40.
The grip is very wide to hold that magnum size round, so make absoloutely sure you can hold it before you buy it. I love mine but my older brother has smaller hands and when he wanted to fire it he had some control issues even with a good two handed grip.
10mm ammo is Heavy. Full loaded with a round in the chamber my G20 comes in at just a hair under 3 pounds (46 ounces: I have a lasermax sight in it and a buttplug that adds a bit more weight but i love both) Even though it's a compact and isn't really an apples to apples comparison, my G19 comes in a full pound less, and carrying a spare mag (another pound) may make the difference between strapping it on or carrying something else.
The extra wide grip makes it damn near impossible to conceal well, and even with really good holsters mine still prints unless I layer shirts or wear a jacket. I don't open carry much unless at a range or an event of some kind so this may be a concern for you.
Hope that helps, I'm the only one I know that carries a 10mm and whenever anyone asks why i picked that caliber, all I do is pop a round out of the magazine and sit it on the table.
