I've said it before and I say it again: magazine disconnecter "safeties" are inherently unsafe.
I will never buy another pistol so mal-equipped.
They're only unsafe in the hands of those lacking the mental facilities to learn the proper handling drills for that particular firearm.
I've owned BHP's for over 30 years now. I carry one with a CCW permit when down in the US, which is a good part of the year - which means I'm statistically likely to have been carrying a loaded firearm with a magazine safety longer than any ten Canadians who have never had an opportunity to carry a firearm unless they were at an approved range and all the restrictions that goes with that. I also carry a BHP on an ATC - where again, I'm probably putting in more time carrying and handling a loaded firearm with a magazine disconnect than any other ten Canadians who are restricted to ranges.
In addition, I've carried a BHP throughout my service in the CF and taught the weapon as an instructor in the battle schools. I don't see a rash of ND's with the weapon, which is something I would certainly seem to expect if indeed it was "inherently unsafe". Nor have I seen anything that would suggest there are more ND's with Brownings than with the C7A1 or any of the other personal weapons we carry around loaded all the time while on tours.
No ND's on my part, nor have I ever seen one despite all the range time teaching this weapon.
So in fact, if I may muse a bit, I've probably got more hours actually carrying a loaded BHP, complete with magazine disconnect, than many of our younger American friends carrying handguns without the magazine disconnect and busily criticizing it while they have little or no experience with it.
In my experience all the talk about the dangers of magazine safeties is a nonexistent problem. However, the solution is simple: for those who find these concerns preying on their mind or fear they are incapable of safely handling this weapon, just don't buy one.
Oh... don't join the CF either: they expect you to be capable of being taught to handle the Browning safely, including its magazine disconnect.
As to the topic at hand, if I had my choice of an M&P with or without the thumb safety, I'd choose the thumb safety option. An M&P without a safety won't fire as long as you don't pull the trigger of course. But then, neither will your 870, Remington 700, AR-15, etc - but we don't seem to have a problem with having and using the safeties on those firearms.
The real safety is the space between your ears, but I am also personally in favour of mechanical safeties as well.
Others, of course, may feel differently.