If you know how to clear a jam on a semi....then dont go with anything less then a .40 caliber, if you are not proficient then go with a 38 revolver or greater.
Next important thing is the ammo you carry. For my duty weapon I carried Golden Sabre, it does terrible things to body tissue. Some like Black Talons and of course my favorite personal loads are Gold Dots.
BTW......when you are talking semi's.......there are only two that are worth mentioning....
1. H&K
2. Glock
In that order of course.
Everything else is second class.
H&K USP's are good, but very bulky, even their "sub" (I forget what it's called) is still pretty big, comparitively speaking.
I don't see how you can reasonably call a Sig "2nd class" though, they're a very fine handgun.
Glocks, you either love or hate, IMO. Personally, I can't get used to the Glock (1.5 action) trigger, but, obviously, lots of people love 'em.
I owned the H&K .45acp when it first came out. I thought it was nice until I bought my Glock 21.
I have changed the pull weight on the glock a tad. Practicing dry fire with the Glock is no fun at all, I use my Springfield 1911loaded model for that. Shooting the Glock though to me is extremely smooth and I can shoot it better than any other. Agree however both being double stack full size guns are bulky for personal carry.
Glock does make personal carry guns that are better suited but I have never shot them nor can I speak of their accuracy. My Glock is extremely reliable and the clips are top notch, unlike my Springfield which I had to spend a little extra money to get top quality mags for it to feed well/reliable.
Sig has a great reputation but I would also recommend looking at the Springfield XD model if you don't have anything against "plastic"
No matter what you get, get out to the range and practice often. There is no substitute for practice. And just like motorcycling practice correctly. I see so many people at the range who can't shoot worth a lick and they just keep shooting and shooting without ever asking anyone around them what they might be doing wrong.
If you get someone to teach you correctly to start with and you listen you will be shooting very well from the very beginning, if you don't you will never get it right on your own.
Breath, relax, squeeze dont pull, and most of all don't push or flinch when you fire. I try to teach people with my revolver to shoot because I can load only 3 rounds and skip a few chambers. If they jerk when no bullet goes off it can show them immediately their bad form. It is something I also continuously go back and do for myself because high powered guns after a few rounds can cause you to start to flinch. This is something some may argue but I keep both eyes open when shooting handguns. I try to concentrate on the target and front sight and that is hard if not impossible to do with only one eye.
Also if you are lucky enough to have a place to practice target aquisition, do it. Shooting at a fixed target all day will never prepare you for something that is moving.
Good luck... Have fun and hope you never have to use it in real life...