Have had both, but I prefer the Shield for several reasons:

The Shield has a safety that the Kahr does not have.
The Shield, being S&W, will have a better selection of holsters available when the dust settles.
The rails on the Kahr polymers are not reinforced with steel.
The Shield trigger, a personal preference issue, is like the triggers on the other M&P guns. Aftermarket APEX trigger kits for the Shield make it even better than it is from the factory.

Gave my CW9 to my son. He likes it, but likes the Shield better.

Both of our guns have CT chin lasers on them.

Have had 3 Kahrs and 5 M&Ps at one time or another over the years. The M&Ps will win hands down 90% of the time for me. If you have an interest in another great, but underappreciated gun, try the Ruger SR9c. The Shield is slightly thinner, but the SR9c is a more robust gun than either the Shield or CW9.