There is also a difference in the triggers I thought. The original M&P doesn't have the same trigger as the Shield. The Shield has a very smooth and crisp break and reset. The reset you can really feel and even hear. I thought the original M & P s didn't have this.

I had heard that S&W was going to change this going forward in the M & P line and replace the older trigger with the newer Shield design. Don't know if that has happened yet or not.