I own both.
The V6 has a punchy mid-bass, but the bass rolls-off quickly and sub-base is small. Its sound is good overall, especially for the price, but it can quickly fatigue with music listening. Th V6 is better for monitoring (especially voice/speech monitoring) than for enjoying music.
The M50 has a much fatter bass. The bass goes deeper, and all bass frequencies are more prominent, to the extent that bass intrudes somewhat on the mids. Despite this, the V6 has slightly more mid-bass impact than the M50. The M50 is far more enjoyable for music listening and far less fatiguing than the V6, but I'd rather have the V6 for speech monitoring. The M50 is slightly harder to drive than the V6; even a modest portable amp tightens the bass a little. These are good, likable headphones. They're perhaps less of a good value for their price than the V6, but more enjoyable.
The M50 is the better headphone, but I understand how some people might prefer the V6 for certain uses.