That Glock you have is an excellent choice. I carry Glocks, they just don't let me down. If you practice with it and get good with it you are good to go. Only reason I'd replace it is if something is wrong with it or if you don't feel comfortable/confident in it for some reason.
There are certainly a whole long list of weapons that would be underneath this one if that is what you are asking. You can do a lot worse, but really not a whole great deal better. Glocks are reliable and they all come in suitable calibres. If for some reason you happened to prefer .40 though, all you have to do is get a .40 Glock barrel and the switch is as simple as putting that in instead of the other.
I read somewhere when they asked some police department to describe their .357 Sigs, they said, "it was like the bad guy was hit with a lightning bolt."
I use 125gr. Speer Gold Dots in a .357 magnum, and I use Gold Dots in 10mm too. No reason to think they wouldn't be a good choice in .357 Sig. Less chance of over penetration if you use a good JHP (this one is) and in a lighter weight (125gr. is for you).
It is basically a hot 9mm and as for how loud it is, all of them are going to be loud. And the shot you fire indoors in defense of your life is going to be the loudest shot you ever hear no matter what you shoot it out of.
Having a shotgun is a good idea too, but be careful you don't get shot with your own weapon. If the bad guy is worth his salt, he'll be on you before you even have a chance to do much considering the situation you are in (behind a counter dealing with the public). So keeping it on you may be in your best interest if you are by yourself.