P9 Covert owner here. Love it. Flawless reliability with a variety of rounds (able to accept +p rounds unlike the Rohrbaugh). And I bought mine cheaper than I could get a PM9 at the time.
It has a slightly longer barrel and slide than the PM9, like maybe 0.7-1". And it weighs 1 oz more. It has the traditional recoil set up, instead of the Seecamp double spring.
I put a Hogue Handall Jr. grip sleeve on, and wrapped an AGrip on top of it to fit my hand. Makes the gun tremendously soft shooting. Very easy to practice with, unlike some other pocket guns. Plus the Agrip is velvety and prevents pocket snags if you only used the Handall Jr. rubber grip sleeve.
I like the longer slide and barrel of the Covert. Longer sight plane should mean better accuracy. Longer barrel should mean more velocity.
I use an Uncle Mike's No. 3 pocket holster. Fits pefectly. I find that the extra slide length does not impede concealability. The Covert has the same grip butt as the PM9, and the grip butt is the key to pocket concealability IMO.