I think the standard P9 is just a 7+1 capacity, but I could certainly be wrong. I think the 'Tactical' series Kahrs in 9mm hold 8 rounds and have a bit longer grip than even the standard P9.
Basically, the PM9 has .5" shorted slide and barrel and a .5" shorter grip. Reports are mixed, but folks with the 3.5" barrel seem to report a bit more consistant reliability than folks with the 3.0" PM9 and MK9. And the .5" longer barrel helps keep velocity up just a bit more than the 3.0".
In my opinion and in my experience, I would prefer to carry a Kahr on my hip than in my pocket and you aren't likely to notice a .5" longer barrel, but you may find a .5" longer grip slightly harder to conceal so they make the P9 Covert... It combines the longer slide from the P9 and the shorter grip from the PM9 but seems to be one of the lesser popular models. I had one which I daily carried for almost a year and loved it.
They also make a TP9 which is the same size slide as the tacitcal series which has a full 4" barrel but has the the P9 grip with its 7 round capacity.
Hopefully, you'll enjoy your Kahr and keep it forever but if not, one thing to keep in mind is that the PM9 is likely to bring a higher resale value than the P9... at least they do around here. HAving said that, unless you were primarily considering pocket carry, I'd go for the P9 or the TP9 if you wanted a poly Kahr.
