Yes, very different... though my time with the Nighthawk has been very limited and my time with HD700 has been very extensive. I am hoping to keep my HD700 but get rid of about a dozen other headphones, and then buy the Nighthawk as it is my favorite of everything I have heard to date. They are very different and I predict it will be a fairly long and confusing transition between the two (weeks rather than days). You will probably miss the HD700's great transients and crystal clarity, percussive snap, plucked strings, spacious and airy sound. You will probably not miss the treble spikes. You will hopefully love the fuller bottom end of the Nighthawk, the equal but somewhat more liquid midrange and the more honest but perhaps a bit rolled off treble. I rate the comfort between the two of them as about equal, but if my ears were slightly smaller and never touched the inside edges of the HD700 I would probably give the nod to the HD700. The Nighthawks were shockingly comfortable to me however, much better than I would have guessed. I like the sound signature of the Nighthawks quite a bit and I expect that I will not be skipping over songs or switching headphones to prevent fatigue nearly as often as I do with the HD700. But when the HD700 is on point, it is a great experience and I prefer it to the HD800 (though I would imagine I would prefer the HD800S).
If you have a nice desktop rig, try to keep the HD700 as it will be a great compliment to the Nighthawk and they both will cover just about any mood or music you may be into at any given moment. If you are going more portable or minimalist and want a more accessible headphone that can be driven by just about anything, then switch to the Nighthawk (assuming you find favor in it's sound signature) and sell the HD700 and don't look back. If your wife is often telling you to turn down your headphones because you are often in the same room and other things are going on... consider the Oppo PM-3 but still give the Nighthawk a chance as it is much less intrusive than a pure open headphone like the HD700.