O.K., I'll go the other way and say get the 40. Without getting into knockdown power, one-shot-stop numbers or any of the other subjective areas the 40 is, without possibility of argument, a more powerful round than the 9mm. While the 9mm is a perfectly legitimate SD round, I'd suggest the 40 because:
1. You don't have oneThe only disadvantages to the 40 are:
2. The guns are virtually the same size. The 40 is .46" longer, .12" taller, .04" wider & 1 oz. heavier (from Kahr's specs)
3. Since this is a single stack magazine and rounds are limited, I'd prefer the more powerful round.
1. More recoil. Snappy, but not un-manageable.So, I'd buy the 40 but it's your money and your gun. But I do love spending others people's money.
2. Slightly more expensive ammo. ATG currently lists Federal LE Tactical Hydra-Shok in 9mm for $25.95 and for the 40 at $26.95.