Shield, Shield, Shield....
9mm Shield would be a good step up. .380 can throw a 95 grain bullet at a little over 950 fps, while a 9mm can throw a 115 grain bullet at 1300 fps or a 125 grain at 1200 fps.
The Shield has a great trigger and is very comfortable, even fun, to shoot. It is very slim in width.
I won't know the Sigs and others that people are suggesting, but I'd expect a lot more turnout for the Shield.
Note: Better bullets have given the .380 acp better standing as a serious SD weapon. Of course this carries over into other calibers, too.