is the k240s better from k240m;

It depends on what your definition of "better" is.

I think that the 240M is a more accurate headphone and has a flatter response curve (to my ears).

The 240S is a louder headphone (due to the lower impedance) and also has a touch more bass. It's also slightly less expensive than the 240M and has a detachable cable.

To answer your question...if you want the most accurate headphone the 240M would be the best. If you want the loudest headphone and like a little bit more bass the 240S would be the best.

Really BOTH headphones are pretty similar...unless you are mixing with headphones I'd buy the 240S for general purpose monitoring.

That's just my personal non-expert opinion.