I'd go with the 40s&w. I do not own any polymer frame Kahrs, but I do own five steel frame Kahrs and counting. My next buy will be a T9 and T40. I currently own two K40s and one K9, MK9, and MK40.
WEIGHT AND SIZE:
What I find odd is that many will ding the MK40 and the K40 as being too small and light for the calibler; however, the Glock 23 which is the 40s&w size and version of the Glock 19 weighs LESS than the MK40. That takes care of the weight, but as far as size goes, you can get a full grip on both without sacrificing concealablity via the use of a pinky extension. To recap, you can get a full grip and the MK40 weighs as much as as some double stack Compacts.
RECOIL AND HIGH PRESSURE:
40s&w gets a bad rap because people regurgitate what they hear.
This is a chart of the most popular and common 9mm self defense loads. Look at the velocity numbers and expansion diameters.
This is a chart of all the most popular and common 40s&w self defense loads. If you compare the velocity between the two, you'll see that in manu cases, the 9mm loads have higher velocities. What is also funny is that people who shoot the light weight only 16 once polymer Kahr CM9 or PM9 will probably experience MORE or similar recoil in comparison to the 25 once all steel Kahr MK40, yet they will tell others to stir away from anything 40cal.
Here is 45acp. If you compare it to the smaller diameter 40s&w loads, the velocity is only marginally lower, about the same, or even higher depending on the manufacturer and load.
My point is, there is no set in stone rule that 40s&w is more snappy than 9mm or 45acp like many lime to claim. It all depends on the type of powered being used, the number of bullet grains, and other factors aka it all depends on what ammo you go with. With the popular 180gr Federal HST round in 40s&w, you get a velocity similar or less than 9mm, with better intermediate barrier penetration, and larger expansion. With the popular 180gr Federal HST round in 40s&w, you also get similar or ever so slightly more velocity as 45acp, but you still get decent expansion, good barrier penetration, and more capacity.
CAPACITY:
The MK9 is 6+1 (7) or 7+1 (8). The MK40 is 5+1 (6) or 6+1 (7). There is only one round difference between the two. The MK40 also accepts the 7 round Kahr T40 magazine.
AVAILABILITY:
The last and final reason why I suggest that you get the MK40 is that Kahr is doing away with most of their 40s&w pistols. You might not get the opportunity to easily buy one NIB in the future. Get the MK40 now being that you can always get a MK9 in the future if you so choose.