If the CM40 was just announced, I'd wait at least 6 months to let them work the bugs out.
I've had several Kahrs, all 9mm.
With quality ammo, the difference between the two calibers is negligible.
I've had my current CM9 about 6 months and love it.
No jams or issues in 300+ rounds.
I carry Hornady Critical Defense ammo in it, but it feeds Remington and Speer hp also.
I don't think you could go wrong with either choice, but I'm betting the 9mm will be easier to handle in such a small package.