HE-500 = very balanced, everything sounds right...definitely can't go wrong with the HE-500s, not as comfy and a bit on the heavy side
HD700 = fun sounding, larger sound stage, very detailed, EXTREMELY comfy...words cannot describe the comfort level of these headphones. The comfort factor alone should put these in the summit-fi area. I think the only issue is that it may need a warm sounding tube amp to mellow down the treble spikes.
Personally I don't think you could go wrong with either one...they both compliment each other very well, they're both high end headphones, I think it'll all come down to personal preference.