I have the K9, MK 9 and PM 9. So I can't compare the K9/P9 but I can their somewhat shorter little brothers for the steel/polymer question.
The steel IS different than the polymer. Better? Maybe. Probably. It is definitely easier to shoot and I get consistently better results with the steel. Which is not to say I don't care for polymer or that I am not open minded about polymer.
For pocket carry, I definitely prefer the PM over the MK, although I have pocket carried both. In this application, polymer wins and wins big. I am more than willing to sacrifice the steel advantage for the weight reduction in this arena. But for belt carry, the MK is really quite nice.
I consider the K9/P9 size a bit too large for pocket carry (although I have done so). Which means belt carry. I don't think that the lighter weight of the polymer amounts to a significant advantage with belt carry. You just don't feel the "extra" weight of the steel with belt carry with a good belt and holster. So I would stick with steel and its advantages.
I prefer the K series over the MK series for the ergos......the natural point, the feel in the hand, the joy of just being able to shoot something nice....really nice.....not that the Kahrs are the only way to get that.....
You can also get laser-grips for the steel series, if that is something that appeals to you.
Maybe I am among the lucky ones, but, after break in, all have performed just fine. I recall having some minor issues with only the MK during break in. After break in, hunkey dorey.