M/S2 is the replacement for the M/S. The tread pattern is very similar, but the sidewall is visually different, and the 2's are an improved tire. I figure it's only a matter of time until the M/S is completely discontinued so I'd get the newer tire in case one gets an unrepairable hole.
I've had two sets of M/S in the past and the M/S2's keep up the good name and come with a 70,000 mile warranty.