The HE-500 has better imaging, detail levels, treble detail+extension, better sub bass and lower distortion overall. It has a far faster response and is a fantastic all rounder. The HD650 is more laid back and has a fair bit less treble and liveliness. The 650 is more romantic "euphonic" but the HE-500 is still very enjoyable for mellow or sultry music.
I thnk the HE-500 has super imaging overall, but the HD650 has a slighty better center image, all IMHO.