The .40 may have a small advantage in power, but you can put more 9mm rounds in the same sized pistol...with the result being essentially a tossup...shooter's choice.
If you do some searching, the Kahr PM9 is at the ragged edge of firearm design. Most of their 9mms work...in the same sized frame, many of the .40s do not. There is a practical limit for everything.
BTW, it's not .9mm...it is 9mm.
.9mm would be somewhere around .032, thirty-two thousandths of an inch. Slightly larger than the needle the nurse uses to take your blood or give you a flu shot. Probably not very effective as an anti-personnel round.