First, I would post a question on its own thread in this section (or in the "Introduction and Help" section) asking "Help! HE-560 or HD 800? Who Has Both?"
I only have the HD 800, and though I have the HE-500, I don't have the HE-560. No one in the data I analyzed, which ended before the HE-560 came out, had both, and I really think you need the opinion from someone who has lived with both for a while. Shoulldn't be too hard to find someone like that, as most people who will buy the HE-560 are likely to already have the HD-800.
The HD 800, and to a lesser extent the HE-560, is very dependent on how it is amplified. There are a handful of good amps that folks agree are great for it (I have the one that Sennheiser built for it, the HDVD 800, and I'm Very satisfied). But there are other folks that have other than ideal amps who complain that the HD 800 has fatiguing treble and no bass. I think both of these concerns are mitigated by the right amp.
So you need to see what type of amp the person comparing the HD 800 to the HE-560 is using.
Here is another thing you can do... go to the "Rank The Headphones That You Own" thread and do a "Search This Thread Only" for the HE-560, on the chance that many people that report a ranking on it will also have the HD 800 and give a ranking.
For example, for me that unearthed this post, in which the poster places the HD 800 over the HE-560:
http://www.head-fi.org/t/109756/rank-the-headphones-that-you-own/4560#post_10949351