To my ears the M50 and M40f/s have a pretty drastic amount of difference in sound.

For clarity; I'm not saying the M40's sound 'better' than the M50's. That is not what I am after. I think a mix done on M40 f/s transfers better to other systems than when I attempt the same mix on M50's (M40's appear to be more true). I may be unique in that opinion, but I have done side-by-side comparisons here using those two.

If I was looking for a set of headphones to give a client to listen thru to make the work sound 'better' I may choose differently, but that's a different set of parameters.

I do not work here, but the benefits are still awesome
Make your sound your own!