The PM40 & the CW40 are the exact same thickness. The PM40 is about an inch shorter in OAL, and about 1/2" less in height. It holds 1 less round of ammo. The PM comes with 2 mags, while the CW comes with one.
Can't address reliability specifically for those 2 models, but I suspect it's good. My P40 has been 100% reliable.