I suppose it depends on how much listening you intend to do, and how much monitoring.
The D66 is a superb listening phone - slightly crippled by it's comparative openness when outside, but still damned good - and what's even better, it sounds good without an amp. Unfortunately it's also pretty fragile - although nothing bad's happened to mine so far, and after having taken them apart, I see they're pretty easy to fix.
The HD280, while being as I said superb monitors, are not so hot when it comes down to just listening IMO - the response is too flat. It works reasonably well with Jazz, Classical, etc but even in those genres a bit of low-end meat on these cans wouldn't hurt the overall rendition. I certainly don't sit down with an album with the HD280.
For a while after I bought the HD280 based on recommendations here, I was really disappointed because they just didn't have the sound I wanted, especially for the majority of music I listen to. And since I bought them for outside use, the looks... jeez, the looks. There was all sorts of arguments about whether they had enough bass or not, and whether they looked ugly or not... But one day, I was editing something I uploaded from a Minidisc, and I used the HD280 instead of my JBL monitors. It immediately clicked and I've dedicated the 280 to this purpose since.
