Not to bump an old thread again, but I'm guessing people will continue to find this since it has 6XX in the title.
I also found the comparisons between the 58X and 6XX confusing and inconclusive in the past.
I'll add to the mix by saying I own both of these headphones, and I really don't like the 58X. They sound grainy and gross in the mids or treble. I'm not smart enough or equipped enough to quantify it further. Maybe it's to be potentially expected based on the graphs in this thread, or maybe there's something about my particular pair that I find disagreeable. Or maybe I'm crazy. I'm not sure I can know. But I don't like them. Meanwhile the 6XX is my favorite headphone.
So I obviously recommend the 6XX. Even setting my personal preference aside, I don't think you can really go wrong with the 6XX, assuming you're looking for a general, all-around, solid, relatively affordable pair of headphones and not trying to become a Crazy Headphone Person. The 6XX is very popular. It's well liked, well known, well supported, quite thoroughly discussed and understood, etc., especially considering its close relation to the HD 650. I think that all counts for something.