I have the Nano and P290RS, and have had the PM9. All fne little 9mm pistols, but none of them fit in the front pocket of my Dockers, are as light, and are as easy to pull as my LCP.
Now, somedays I do want a little more oomph in my pocket, and carry either the Nano or the P290RS. Of the two, the P290RS is shorter from top of sights to bottom of magazine, and comes out of the pocket a little easier. I also like the fact that the trigger pull of the P290RS is a little heavier, thus safer for me in pulling it from the pocket in a hurry. The fact that it comes with night sights doesn't hurt. They are available for the Nano, but close to another hundred bucks.
The Nano is nice, but prints a little more in my pocket because they didn't finish the corners of the slide as well as they could, and are a little "sharp".
One area where the Nano does shine for me is that it seems to come up right on point very naturally. Some call it a high bore axis, but it just seems to come up for me right in line with my intended target better than any other small auto I've tried.
The PM9 is about 3 ounces lighter than the Nano and P290RS, and was also a fine pistol. I got rid of it for reasons other than quality. It did have a wonderfully smooth trigger, like most Kahrs I've tried.
Bottom line is that the little nines are just that, but they are nowhere near as small and light as the little .380s. (I've also had the BG380 and P380.)
It's ultimately up to you to determine how much power you can live with.
I'll just say that for me a pocket pistol is always a second gun, backup to something else. On the rare occasion when I can only carry a pocket pistol, it's a Ruger LCR in .357 magnum. If they ever do actually come out with the Nano in .40, like they've been saying for a long time, I will defnitely get one, and it will be added to the LCR as a "pocket primary".
Good luck in your quest.