For me, pocket carry is all about size, the smaller the better (in both weight and physical size). I notice every extra ounce of weight, every extra 1/10th of an inch in width and length when I'm carrying a pocket gun in loose fitting summer shorts. I even consider my 13 oz J frame unacceptable as a pocket gun, and same with a PF9 I've tried. (maybe I have small pockets?) So yes, I can completely justify a 9 oz pocket .380 as a better choice than a PF9. If it's not under 10 oz (empty), it's off my pocket carry list. I know guys who seem convinced that they can comfortably carry just about anything they can wedge in to their pocket. I'm not one of those guys. If I'm carrying IWB, then I bring the 9's out of the safe, or better yet, my DAO J-frame.
As far as ammo cost? I shoot my LCP just often enough to maintain a feel for it. I use very little .380 ammo. It's not my range gun, so ammo cost is not a factor. A P3AT and LCP are those guns that fall into the category of : carry a lot, shoot a little.
And by the way, during those hot summer months, it's not about which one I can shoot better (my LCP or PPS for instance). I shoot the 9's or J Frame better. Its about which gun I will have with me. That's all that matters when I'm in shorts n' T shirt, or just shorts.