Everyone is different, but I've tried full sized autos, single stacked autos, compact versions of service pistols and small revolvers and have a few conclusions.
I find that the difference between a single stacked model and a double stack model is pretty insignificant, at least for me it. These guns carry more rounds, have a wider grip (important for me), with a grip sleeve, they can double as a home defense gun and they have just enough weight to keep recoil manageable...usually an unloaded weight of between 19 and 23 ounces. Not to mention, they can except accessories likes lights or lasers.
So, something like a G26 or other double stacked (P99c or M&Pc in my case) gets the nod for inside the waistband, while easily doing double duty for home defense.
The problem is pocket carry. Something like a G26 just seems to wide and too heavy for my pocket...not to mention the 5.5 lbs trigger pull doesn't make me comfortable when only using a soft holster. (Yes, I know this can be changed.) When I want to pocket carry, I use a very "pocketable" gun...a j-frame revolver.
For me, I resort to pocket carry when dressing "nice", tucked in shirts and something like slacks or a nice coat. Having two weapons in the rotation means that I have a lot of options.
Anyway, the PM9 can definitely do pocket carry or IWB, but if you're carrying inside the waistband, you can probably get by with a larger weapon.
It's nice to have options, huh? I'd buy the one that I plan on carrying the most often, with the aim of getting a second pistol down the line.