Both are excellent, and will come down to a question of taste. I auditioned the AR5000 for about an hour in a store along with the 600 and another known headphone in the same price category, but not the 650. What impressed me about the 600 is that comfy midrange focus (apparently, the same with the 650). However, the Aune excels in many areas. For one, the bass is fast and every note can be heard, without sounding at all boomy or overpowering. I never knew Supertramp had such a wonderful bassist. The bass is uncanny. The highs are extended without any harshness. In fact, the highs are so pure that you wish there were more - and this is coming from someone who is allergic to highs and boomy bass, or even constant sub-bass. Orchestral music sounds expansive, whereas the 600 sounded more claustrophobic. Wagner tubas sound they way they should (on the Aune) - the glory of the gods.I see the AR5000 quite often compared to the HD600/650/6XX. I own a HD650 and like it. Although these are not the most exciting headphones imo.
Will the AR5000 be noticably different from the HD6's and as such worth buying? Or will it be more of the same?
Also, the Aune is very comfortable to wear, with very little clamp. I wore it this summer and was never bothered. The clamping force is very light and cushiony.