I just got in the 880 600ohm version and am very happy with these. I recently sold a pair of the HD600s as Sennheisers, to my ears, have a critical flaw in the upper bass/ low mid range (?). (I found this even into the HD800). It's a subtle smearing but its in a bad place and for me ruins the experience of what otherwise is a very good sounding headphone.
To my ears, the 880s are much more balanced sounding. I had the T1s for a little while and these are quite similar (but not the same clarity). The presentation and scale is quite the same.
I would suggest the 880s over the HD600 because of that annoying frequency band in the Sennheisers. Both are good though.
I've read that the 880s are supposedly bass shy. Sure they don't rumble like the Denon D2000s but they are quite good in the bass and I think a little more articulate than the HD600s. The uppermids are where I think I might prefer the HD600 but not much.
Overall, the 880s perform better across the board. (to my ears, of course)
Also, I have a large head and found the HD600s a little tight, though I guess you could bend the headband. The 880s fit me better.
btw, I'm not sure what you are planning for an amp but I suspect that both need a good amp. I was using a bottlehead crack with very good tubes.