I could never make this decision. To this day, I’m enamored with both the HD700 and HD800. (Actually, I’m still quite fond of the HD650 and HD580 too).
When I first auditioned the HD800, however, I was dubious; I just didn’t know how to mentally process that amazingly immersive soundstage, but after about a year (and some improved gear), I finally had that special moment with the HD800 (call that moment an epiphany, if you will); afterwards, all the imagining and sound-staging idiosyncrasies morphed into something breathtakingly holographic.
Today, I really can’t imagine being without the HD800. There are times, however, when the HD800 is just too much of a good thing, and I crave a more traditional headphone experience, and that's when I reach for the HD700; it takes a moment to readjust to its smaller soundstage, its more terrestrial imaging, but when I adjust, the HD700 always thrills me. Its immediacy, its taut and yet impactful bass, its PRAT… Ugh, what can I say: I just love my Sennheiser headphones.
Oh, btw, if you mosey on over to the HD800 thread anytime soon, be warned; there’s some drama over there right now.
Just for info, you didn't find your HD-700 too cold/analytic and/or bright/harsh compared to the HD-650?
Brightness and harshness/sibilance is always what I'm afraid of coming from the 650 (like the Beyerdymanic T90, ouch!)