Oops... I feel a little late on this one...
Right... the bad news... they both have (Ultimate... i.e. when compared to full size gear) compromises...
The DT250 bass is snappier, but sounds thin... not punchy, the mids are thick... pretty cool for vocals, but could smother dance music very easily... and the treble is very recessed (although that is good for sibilant tracks)
The D66s... ultimately boxy bass, with a slight emphasis to the midbass area... the midrange is thinner than the DT250s, but to me it is more neutral... easier to get along with... and yes, the treble could be classed as sharp, but only in an a/b between the beyers and the sonys...
In terms of portability, its a one horse race... the Sonys walk all over the Beyers, in both their effeciency, and also their sound quality... much more involving, and that cleaner treble in the sonys means that if you're on the bus or something that outside noises aren't as apparent...
for full sized equipment, the sonys falter a bit with that boxy bass, and the beyers, with more power on tap sound a little better, but they still have a very snappy, yet light bass when compared to the Sonys... and whilst the midrange is forgiving, or maybe even enticing for female vocals... that lack of treble is ultimately aggrevating...
but as i said at the top... this is when both are compared to the Sony CD1700 / Philips HP890 which are both a lot bigger, and have more capacity to enthrall you with their sound...
Both the beyers and the D66s are pretty good at what they each do best... Sonys for portable (they're still in my top 3 of all time favourite portable 'phones) and the Beyers... for soft female vocals...
If you have a particular taste in music... use a particular source (PC / PCDP / full setup) let me know, and i'll try to compare on a setup similar to your own for a more authorative comparison
Thanks
Duncan