Welcome to the forum and congrats on your Kahr. There is a extended mag for the MK9 that enables you to rest your pinky on it plus it holds an extra round.
The K9 is only slightly bigger and will give you increased control and should give a little bit better accuracy with the 1/2" longer barrel.
I have a MK9 and thought about getting a K9 as well but I bought a S&W 3913 pre-rail and i'm glad I did. I'm sure the K9 would have made me just as happy.