I really like the LC9s. The new striker-fired version has a MUCH better trigger than the original LC9. The only things that I don't like about the LC9s are the safety and the stupid magazine disconnect. I hate magazine disconnects, and generally don't mind having a safety on a carry gun, but this one seems kind of sticky and difficult for me to actuate. NOT good on a carry gun. However, I see that Ruger has a new "LC9s Pro" out that gets rid of the safety and the mag disconnect. (It still has the Glock-style "safe action" trigger.) I got my LC9s just before the LC9s pro came out, and I may trade for that one.
I think the obvious choice is the Sig Sauer P938, surprised it hasn't been brought up. It's 9mm and basically a slightly larger clone of the Colt Mustang that you currently carry.
This is the first thing I thought of when I read the OP. If you are used to a Mustang why would you not just go to the Sig it's basically an improved mustang clone. Heck the mags on the 380's are interchangeable... The 938 shoots great and you wouldn't be disappointed. My pocket gun options would be that or a Kahr... However I would much rather carry a shield on my belt for summer time carry.