I asked the same thing last year, and all I could get out of the Heidenau information line was M&S stands for Mud and Snow. I have no idea what difference that makes, but I decided to opt for that tyre since I was the most expensive and therefore likely the longest lasting.
I've had them on my F800 for 4000 miles and reckon they're about half way worn - an 80:20 mix of road and off road.
As for speed rating, by law I think you have to get a rating at least as high as your bike is capable of. I went for the T speed rating as that is good for about 120mph - as much as the F's will do.