I would recommend that you go to a range and rent some of the guns you are thinking about. This small expense may save you from getting a gun that isn't as comfortable in your hands, or doesn't shoot naturally for you. Remember that the more you like your gun, the more likely you are to actually carry it.
That said, I love sigs, but for the new shooter I would recommend checking out the new S&W M&Ps (9mm and 45 are my favorite calibers, but most people are fans of .40), or the Springfield XD (the 45 xd actually has a higher capacity than the 40 xd!). I really, really like the springfield XD 45. If I were to recommend a gun to a new shooter that wants a .45, that would be the one.
A lot of people will recommend Glocks, but remember that they don't work for some people. For me, a glock does not point naturally, and I always have to adjust my point of aim when I draw. The XD on the other hand always lines the sights up naturally. Same with a Beretta. This is different for everybody.
A nice new gun on the market to check out, if you aren't interested in a full size service pistol, is the new Taurus 24/7 compact. It's actually very comfortable to hold and fills out the hand a lot more than you would expect by looking at it, and it is a lot easier to conceal than a full size pistol. Remember that you are sacrificing control for size whenever you go small, but you are also making a sacrifice of concealability when you go large (this can be corrected with an appropriate belt and holster. BELT and holster. The belt is important)
Good luck! And btw, it isa greatfeeling to know that you can take care of your own safety without expecting the police to, so congratulations on making thegood decision to go armed.Just remember the wise words ofPeter Parker's uncle,"With great power comes great responsibility". Be responsible and learn the 4 rules. Use common sense and be a good person. That's all of my advice and wisdom, so welcome to OCDO!