I haven't heard them, so someone who owns them can feel free to correct me, but if the 440s are set up at all like the 840s they'll have more midbass punch and a more forward midrange. The HFI-680's will give you more treble and a lot more sub bass, and sound sharper and more expansive, but they're also a little sibilant.
I'd lean towards the 440s for rock and vocals, and the 680s for electronic, but you shouldn't be disappointed with either.
Although I listen to rock more than electronica, the expansiveness on 680s generally sounds more appealing. Only problem is I hate sibilance. Plus Ive been reading that for a normal sized head the clamping can be pretty heavy... and my head is decidedly on the large side of normal.
Trying to find places to audtion them, but so far only found the 440s not the 680s. Cant find anywhere around Melbourne that seems to stock Ultrasone...