Most everything that can be said, has been said, concerning this situation.
Training is important -- critical. Lots of civilians buy a gun and don't get the training, though.
Using the take-down tool on the M&P is a virtue but not a perfect one. It takes time/patience to extract the tool, and many prefer to pull the trigger to let the slide go forward. Which is back to square one, the same as the Glock. I prefer to use a pen to push the lever down, or struggle to get my finger in there, rather than pulling the trigger.
If you use the M&P system -- and if I were a police department, I would teach everyone to use a ballpoint pen and tell them they are in big trouble if anyone ever sees them pulling the trigger to let the slide go forward -- you are in a safer place than if you have a Glock and use the Glock system. This will be important to teach if the Armed forces go with the M&P to replace the M9 and M11. GI's will find the shortcut (just pull the trigger!) and the correct way -- use a tool through the top of the slide -- will have to be drilled into them. (I know. I were one).
Glocks, by the way, also have more then their share of NDs while being put back in the holster (that's right, with the finger in the trigger guard).
As an aside, I really like the grip safety of the M1911A1 -- wish it could be built into the M&P somehow.
Training, backed up by soundly designed weapons systems... helps cut the odds of a ND...