both are very good units, difference is M12 is modular so you have the option to use only those power cables that you need and keep rest aside. this will make your build look neat. whereas in S12 you can't do that as it is a non modular PSU. The M12 is also slightly louder than the S12.
So if you want to show off your build and are ready to pay the extra, go for M12. But if you are in a budget and don't mind cable clutters, go for S12.