I have indeed owned both the HFI700 (mine says 650DVD, same thing) and the DT770pro 250ohm. I have also tried the 80ohm version, not broken in at guitar center, and the 600ohm version at Dean's place yesterday. SO I have all the bases covered.
When I owned the 250ohms, I used them as portables sometimes, but they were just too damn bulky, though I liked the isolation and short stretchy cord for going portable. The 650DVD is my current portable, and it is much more discreet, and convinient, though the isolation is not as good, and the long cord needs to be tied up (perhaps I will recable some day).
Sound-wise the Ultrasone has the strongest bass in the lowest octave of really almost sub-sonic headshaking stuff, but the rest is not nearly the DT770 which borders on too much bass for me like the Grado PS1. The mids of the DT770 sound better, but they are more hidden by the bass. The mids on the Ultrasone are kinda dry and lifeless. The highs of the DT770 are more extended but harsher with bad recordings. The ultrasone's highs are still sibilant, but not as harsh, rarely annoying, as the DT770's highs can be annoying quite often. Also the Ultrasone is very closed-in sounding whereas the DT770 is one of the more open sounding closed cans I've tried (which is almost all of them out there). The imaging of the Ultrasone is slightly tighter, despite the smaller soundstage.
The DT770-80 has the most balanced bass with the mids, so it is the one I'd want of the three variations, then the 600ohm, then the 250. The bass gets progressively louder in that order, drowning out the mids.
Soundwise I prefer the DT770-80, DT770-600, HFI700, DT770-250 in that order. I still own the Ultrasone becuase it is much more portable, and that is what I need since I have the woodies at home. Feel free to ask any more questions you might have.