I either own or have owned several, including the MK9, MK40, P9 and P40. Still have the MK9 and P40.
The P9 is the only Kahr I ever had problems with, but Kahr finally replaced it.
The weight of the MK9 is not really all that, it's just so small to weigh that much, it feels heavier.
I never carried my MK40 in a pocket, but I guess you could. For me, it came down to the grip length. I'm just more comfortable having all three fingers on the grip, as well as a full backstrap in the palm to fill up the hand. There are days I miss my MK40, but I'm pretty happy with my P40. Just depends on what you want it for... if you carry in a holster, why not get the P series. Even better, don't buy anything until after this year's SHOT show, and see if Kahr unveils the "MP9", which would take care of all your needs. I plan on buying one as soon as they exist.
If you don't want to wait, or they don't offer them this year, there's always the P9 Covert. The slide length is the same as the P9, but the grip length is that of the MK series. It will fit in an ankle holster, since the grip is the part that shows most anyway. Plus you get that polymer light weight you're obviously interested in.