The S series is designed to be used in a seated/stationary environment. The M series was designed to be used in a live/mobile environment. The core technology (power, transducers, circuitry, membranes etc) are the same, but the difference in form factor makes it ideal for its different uses. The S series has a wider surface area, and this creates a somewhat broader experience.
Both will change the way you experience music and sound forever, it just depends what your primary applications are. The S1 and M1 will not be produced anymore to make way for the S2 and the M2 product evolution.← What is the difference between the S and the M series?