I have owned an MK9, K40, MK40, P9, and P40... (sorry, no K9).
My experiences with all the steel Kahrs have been excellent. The recoil of the MK9 is quite mild, so much so that my 11 year old son considers my wife's MK9 his favorite pistol to shoot, now that I sold the KelTec P32 he used to favor.
The P9 (which you didn't ask about) has more felt recoil than the MK9, even with the MK's shorter grip.
My personal conclusion is this... If you're going to try and pocket or ankle carry this weapon, then MK9 is the way to go. If you're only going to CC it IWB or belt carry, then by all means, get the K9 for the longer and better grip!