The TP9 currently produced takes only the 8 round mags, making the difference in height a bit over 1". The longer 4" barrel isn't hard to conceal, but it is the grip that some people would say make the TP harder to hide.
I don't have a PM to compare to, but one can easily assume the longer gripped TP would be the easier, softer shooter of the two. I recently picked up a TP9 to add to my Kahr collection and all I have for it is praise. It has impressed me so much that I've even considered using it to replace my much beloved CCW.
The longer grip, barrel and sight radius could only prove to be improved assets over the short PM in a head to head comparison, IMHO. The TP is certainly the bargain of the polymer line... another asset.
There are some TP9s that take the 7 round mag and obviously have a shorter KP sized grip. If considering a purchase, make sure you know what you're getting.
Judging by today's left wing, looks like Senator Joe McCarthy was right after all.