My first impression by the title of your thread, and using .9mm and .40mm is that you may well need to study, and learn more about firearms before you decide on either.
Now if that was a typo that you didn't catch, I'd say you need to try and find a range that rents guns, has both, and try them. I do carry a KAHR CM9, and it shoots very well. I also have a Springfield Armory XD40 Subcompact that is somewhat larger, and heavier chambered in 40 S&W. I am very tolerant of recoil, but do notice a sharper, and more pronounced recoil and more muzzle flip with the larger XD in 40 S&W over the Kahr CM9. I see nothing in the design of the KAHR that would reduce recoil, so I would think that the CM40 would have considerably more recoil than the 9mm version. I haven't shot a CM40, so I don't know for sure. But my CM9 does have more noticed recoil than my larger, and heavier 9mm pistols using the same ammo.