I added the Shield. Already had the full size and 9c. I picked up the Shield for warm weather carry, found the 9c was a little bulky in shorts & T-shirt season. Much as I like the 9c, (it'll always be my favorite M&P), turns out I carry the Shield pretty much year round. Small enough to easily conceal, big enough to do the job should the worse happen.
Never argue with an idiot in public. People passing by won't know which one of you is the idiot.