Well the WS99 is a portable and is more comfortable than the M500, so not really apples and oranges. And don't get me wrong, I love the M500 (especially for it's sound). I'm not thinking about giving it up, but it hurts my head after awhile. lol...It's fine on the ear though (the JVC S500 is a portable that hurt on the ears).
Then they had to sacrifice sound quality for the WS99s. Thankfully that is not the case with the M500s. Plus every head is different.