Hard to say,
M6 has much better firmware, 10 band EQ, long battery life, plays OGG and FLAC but lacks brute force when comes to video and sound output is not that clear.
C280 is best player (when it actually comes to playin something

) for me, because it has really great sound quality, video quality, but lacks features*.
So when I need features and going somewhere out, I take M6. For travelling and listening somewhere where I can hold player all the time, I prefer C280. It is also more logical, cause my m6 has only 1GB and c280 have 4GB of mem.
I think that meizu did great job with firmware and controls, I only wish if c280 manufacturer could do the same.
* I mean features like playlists, locking slider, format support, continue in track after power off, browsing tracks by artist, album etc. these things M6 knows and c280 not, but as I said, it is only matter of firmware, cause M6 is obviously inferior to c280