I bought the M&P 9mm last year, and can tell you that it is an incredible firearm. 1000+ rounds of cheap ammo through it with zero failures what-so-ever.
I bought mine without a magazine disengage. I felt that in my situation it could only cause problems. If the bad guy gets my gun, I figure I've already lost. From a security standpoint, where your gun may be in the open, dropping your mag if someone reaches for your gun may be essential.
The handcuff key lock is kind of neat. The only use I see it for is for storing the gun. If someone gets their hands on it (innocent child or criminal), they won't be able to turn it on you or themselves to cause harm.
Anyway, I would definitely recommend the M&P 9mm. If you can, shoot as many guns as possible before making a purchase. What won me over was how the M&P felt in my hand. It was like a 1911, but held 17 rounds of 9mm instead
