M12 is modular, this means the cables running from the power supply can be disconnected. The S12 is the silent model, and might also be ever so slightly quieter (probably won't matter in a case).
I personally love my modular Sunbeam, helps a tons in a build. So I'd personally probably pick up the M12. Doubtful you'll need more than it offers for awhile unless you go super high end out of nowhere.
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Skip the above. Did a quick search and saw that the Corsair 520HX is going for $125 now with free shipping for the time being. I'd certainly grab this one over either. Its like a combination of both, modular cabling, one 120mm fan, 520watt, similar amperage. Corsair power supplies are OEM Seasonic's anyway I believe, but come with a 5 year warranty. Better deal, and just as good.
Grab it here.