Mike Irwin is on the beam. He's sometimes a man of too few words, however.
The Kahr 9s and the Kel-Tec P11 all hold 10+1 rounds (sic - see edit below), they easily handle +P loads, and they're small. The P9 and P11 are also very light for 9s.
Briefcase carry is a special category of concealed carry. I presume you want to have a small 9 tucked away there along with all the other stuff (papers, books, etc., which comprise the reasons you need a briefcase in the first place) instead of carrying a briefcase just so you can carry your gun.
That being the case, you're going to want a means to secure it from migrating around inside as your briefcase gets tossed, turned, and (gasp!) dropped. Think of the comedy routines where women are fishing around in their handbags for something or other. It wouldn't be funny if you had to fish around for your 9 if it was needed. It also wouldn't be funny if it appeared in plain sight of some client or nosy stranger.
The compact polymer 9s Mike recommended lend themselves very well to being secured in their own special places in briefcases via some imaginative methods. They're small enough and light enough that they'll stay put.
They're also DAO with industrial strength triggers that don't need extra safety mechanisms. That's also why I'd avoid Glocks for this purpose. A Glock cannot, IMO, be safely carried without a trigger guarded holster.
Think about how you're going to be able to access your BC gun if needed. Time wise, you don't need to be messing with safeties, racking, or cocking. If you have the time, you might just as well want to slip the gun into your pocket. You might need to fire from within the BC. There are many scenarios, and after thinking about a few of them, I'd rather have a DAO ready to go or fit any stealthy rearrangement that seems appropriate.
I'll be curious about what you settle on and your reasoning!
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Edit added:
I KNOW Kahrs are NOT 10+1. The plastic fantastic 10+1 9mm compact class includes the Kel-Tec P11 and the Taurus PT-111, but Kahrs are still worthy candidates for BC carry.