As having both U70 and Form 1 headphones I can comment on the first two:
U70:
-oldest of the three
-big, bulky and heavy. (But when adjusted with patience they fit well)
-original black leather finnish of the earpads has worn off by now, not necessary to replace
-long cable with 6,3mm plug
-needs a lot of power to drive, (fe. iPod needs volume to be almost at max level)
Form 1:
-much lighter and no head-control adjustments, easier to fit
-will need new foam pads for earpads, available from LifestyleAV
-shorter cable with 3,5mm plug
-easier to use with modern devices as it's much lighter to drive
In terms of sound quality I have no real complaints about either, U70s are a bit bass-light in comparasion, though.
The newer Form 1s and 2s are likely technically better specified and easier to come by.
Bang & Olufsen - The art of controlling sound, picture and light