My wife was in the same quandary not long ago. She loved the Glock 19, but decided it was too big for daily carry. She tried the Glock 26 and hated it. She couldn't hit anything with it, and the extended grip pinched her fingers. Fortunately I had a friend that owned both the Kahr CW9 and the PM9. My wife shot both and liked both. My wife could get 2 3/4 of her fingers on the hand grip of the PM9 and decided that was good enough. She liked them equally and could produce very nice groups with both. She decided she'd rather have something smaller than larger and went for the PM9. I would warn that the extended magazine is rather useless (except for adding 1 round )since it does not improve your grip at all as it is not flush with the hand grip.
I was a bit surprised to see that the rear rail slide was made out of plastic. (Both CW series and the PM series.) They have a lifetime warranty, but like others have said, I hope nobody gets hurt on the wrong end of this weapon. If you're worried about this and can stand the weight, buy an all metal version. ($$++) The other problem with Kahrs are they are finicky in loading the first round. The rounds do not sit horizontal in their magazine, but stick nose-up a bit. If you have the action closed and put a loaded mag in it and rack the slide, it most likely will NOT load the first round. Kahr recommends that you lock the action open, load the magazine and then use the slide release to load the first round. If done this way, it will work every time. If you have the action closed, the top round will be pushed out of its nose-up orientation when the mag is inserted, and won't get chambered when the slide is cycled. My wife and I talked about this and decided it wasn't a big deal. With a revolver-like trigger pull, there is no problem carrying this gun with one in the tube--so the first magazine will always function as needed for self-defense. If the second mag is necessary, the slide will lock at the end of the first and you'll use the slide release to load the next mag.
As far as CW9 vs PM9 goes though, it comes down to how well the smaller PM9 it fits in her hand, and if it's worth the extra money for the smaller gun.