It really depends on what amp you are using with them. They are both on the warm side of neutral, very intimate (closed in) soundstage, and very dependable treble. HE-500 is better in terms of extension on both ends. The HE-500 is also great on a budget amp since it naturally has great speed/attack. If you are playing the HD600 on an amp like the Crack, it's up to par with the HE-500 in terms of speed/attack. IMO, the biggest differences between the two (other than build, price and comfort) are the HE-500's better extension (only going to notice if you listen to rap/EDM or have baby ears) and more forward vocals. The HD600 is more neutral in terms of vocals.
I didn't think they were in the same league either until I tried the HD600 on better equipment. On first listen with the Crack, I took them off and looked at my buddy and said, "holy cow, these are no longer the sluggish dynamic headphone that I remember, these sound like orthos."