HE 500--they are more balanced and excellent for vocals. They are also less money.
For information on other cans at or near the top of their line, check out:
Yeah I've read that. I recently bought the HE 400'S and while I love the bass, there is something seriously wrong with its treble. What's the bass like on the HE 500's compared to the 400's?
The bass on the HE 500 is slightly better--but that may depend in part on which pads are used on the HE 400.
It was the mids of the HE 400 that were just a little funky for me vs. the HE 500 (and other cans).