I own the M50 for a year and have borrowed the 660 from a friend for a week
The 660 pad is smaller than M50 and can be uncomfortable if you have large ears and wear glasses.. M50 in the other hand is very comfortable, albeit quite warm for your ears..
The 660 is a bit bright for me, M50 is balanced imho, so the 660 can sound more detailed than m50 when you first try it..
The 660 needs amp, M50 can be "sufficiently" driven by ipod..
M50 staging and imaging is better than 660, the 660 is a bit congested for me
M50 build quality and finishing is far ahead of 660, but 660 is quite sturdy and can withstand a lot abuse.
For me, the M50 is the better headphone, well its triple the 660 ($140 vs $48) price in my country..
Those are my personal experience, hope they can be helpful, and pardon my horrible english..