Well you really have to have both HD700 and HD800 in front of you to judge, I found HD800 lot sturdy and well built than HD700, so in this case higher priced headphone will probably more durable. Keep in mind that HD700 is pretty new headphone, we will have to wait before seeing any failure reports.
As for HD800, there is a problem with the grey paint gradually starts to chip away, or aka "Paint Chipping Problem".
In Canada, HD800 $1599, HD700 $1199, $400 differences. Based on that price I am not so sure HD700 will sell a lot.