They are different, and tastes will decide what works for what person.
HE-500:
Drawbacks - used, meh fit and finish, sometimes bass can be thick (less so with a potent amp with big control in the bass), sometimes on very bright recordings (Yes: Close to the Edge) it can be bright, but, a lot less then most HFM cans.
Pluses: magic midrange, subtle, potent, alive; bass potent and goes into the 30's well, above 2k great harmonics, recording cues, mic changes clear - very. It's a better sound than you get can get from Maggie 3.7i's. I'm using fazor mod, and stock sliver cable cut to 42". Equipment in sig.
Sundara: (note: heard on a lesser system then what my HE-500 sits on.
Drawbacks: many hardware issues (headbands mostly), mids are not magic. Very clean, nice imaging, but its in the clean, but somewhere threadbare of emotions and life family. Both of them crush the LCD2's.
Pluses: New. Very accurate, bass more nimble then 500's.