The biggest factor for me is my overall level of comfort with the gun. That's comprised of how well I can handle it, how easy it is to conceal, what controls/safeties I need to worry about getting used, and the amount of trigger time I have on it.
Perhaps most importantly, there is an underlying subjective factor on whether or not I actually like the gun for whatever reason. There are plenty of guns that I respect and have no trouble with, but wouldn't choose to carry myself, even if I can shoot it well and conceal it comfortably. They just don't do it for me.
Generally, I like small guns, with ~3" barrels, with 5-8 rounds of something potent. I like y triggers a bit on the heavy side, but demand they be consistent; none of that DA-to-SA garbage for me. And bonus points if reloads are easy to manage.
My top choices for the last several years have been:
1. Ruger SP101 3" in .327 - Carried IWB with speed strips, or OWB with speed loaders.
2. Glock 36 carried either IWB or OWB with a spare mag in my pocket.
3. NAA Guardian 32acp carried in a pocket holster with spare mag in opposite pocket.
I'd like to find a Commander style 1911 to become a primary carry gun, but my Paraordinance LDA 1911 has ruined me on the triggers. I like that long, smooth pull on my model, but can't find one in a smaller version than my 5".
“There are three reasons to own a gun. To protect yourself and your family, to hunt dangerous and delicious animals, and to keep the King of England out of your face.” - Krusty the Clown