I have the HD800 and the Aeon... and the CMA600i.
I think ultimately the HD800 is the better headphone, more details, better soundstage, more open, better defined bass albeit not as present. They are both what I'd describe as neutral headphones, but voices have a slightly fuller tone on the Aeon. The Aeon have a more pleasant presentation over all. Same goes for the bass. The Aeons are not a bassheads headphone, but when I first plugged them in coming from the H800 I found them almost bloomy. After getting used to the Aeon I find the amount of bass to be just right and reach for them for pop, electronic, rap and other genres that benefit from a little more bass. The HD800 just brings you a few steps closer to the music, especially on live recordings. They let you see deeper into the recording which is why they are the winner for me. Sound quality aside, the Aeon is the most comfortable of the two, but both are excellent in this regard. The isolation of the Aeon is very good, which of course you won't get with the HD800. I have no concerns about the build quality of either headphone, they are both solid.
Oh and I'll add this. Should you end up with the HD800 you'll have a fantastic pairing with the CMA600I. People complain that the HD800 is too bright and have harsh treble. With the CMA600I none of this is present. The sound is smooth, the highs are never piercing and voices never too thin.
These are just the thoughts on the two headphones on the top of my head, sorry for the unstructured post. If there is anything specific you'd like me to comment on let me know.