The 700's are definitely better than the 600's in many ways. Comfort, sound, imaging, soundstage, separation - they're all better on the 700's. Used, the 700's are ~$500. Used, the 600's are ~$250. The 700's are easily worth $250 more than the 600's. They are much more engaging headphones though, and that type of sound may not be for you. In terms of technical ability, the 700's lie in between the 600's and 800's, and rightfully so are named the HD700's.
I must respectfully disagree, but of course this is only my opinion. for me though, the 700 only excels in comfort. I found the sound cold, out of balance, and unengaging. I might concede though that soundstage is a bit better because of the angled design. If they could have kept the 600 sound and widened the soundstage and separation they would have had a real winner. instead, they just completely screwed up the tonal balance. again, IMHO.
To each his own of course. If you love it, thats all that really matters. The time they spent with me was short lived.