What do you know, a post suited to the two cans i own.
Comfort - I would say they are about the same, just depends on what you like. The 555s are light (plastic), and almost unnoticeable on your head. The DT 770s are slightly heavier, with a metal (for the most part) head band, covered in leather. You can definitely notice them on your head, but its a nice, isolated feel.
isolation - both are opposite ends of the spectrum, 555 offering none (they can get pretty loud at max volume if you listen to them off your head), and the beyers with closed backs offering good isolation (very little leakage).
Sound quality worth the difference? - I have the -80 ohm versions of the beyers, without an amp. Unamped there isn't a huge, HOLY **** THESE BLOW THE 555s AWAY, difference. The senns are great with acoustic guitar and vocals (probably its greatest strength), which may be good for your classic rock. While I like the sound of the beyers overall more, unamped they don't sound above and beyond better.
Obviously with an amp, I'm sure there is great improvement, based off what I have read anyway.
I'm still somewhat of a distant headfier, so i haven't dipped into the high-high end. So my opinion is based purely on direct comparisons of both these headphones as they are the only headfi quality cans I've owned.