Glocks, Sigs, 1911s, OH MY
Well, you stirred up quite a hornets' nest with this question. Not having an overwhelming amount of pistol experience, here are the tips:
-Handle each gun you're considering. Fire it if you can, though in many cases that won't be an option. Check the trigger pull and your ability to manipulate the controls. Through doing just that, I've learned that the Para P14-45, which everyone says has this huge fat grip, in fact fits my hand like a glove, where as the Glock 21 fits very poorly, and therefore I would not buy one.
If you want more than ten rounds out of a .45, your best bet would probably be a Para or something else that takes P14 magazines. Simply because, while Glock 21s and USP 45s do have pre-ban magazines (in 13 and 12 round capacities, respectively) they're in more demand than Para mags and the prices are higher, as well as the magazines being harder to find. On a down note, from what I've heard, it's difficult to actually fit 10 rounds into the castrated Glock 21 magazines, and often they'll only fit 9. Now, if you're made of money and can afford $100+ per magazine for your weapon (and having more than 10 rounds is really that important to you), then by all means, get one of the double columns.
But if you don't care about that, then get a regular 1911, a Ruger P90 or 97, or a Sig 220. All three have 7 and 8 round single column magazines that are inexpensive and easy to stock up on, and you really should have a lot of magazines.