Hi Ewoody,
I have both of these headphones. In terms of absolute sound quality, I think the 250 II are the best. The frequency response is flatter across the entire spectrum and the bass is more convincing at
really low frequencies. The only down side is the "ear sweating" vinyl covered ear pads - I think the better seal that these provide produces the more accurate sound. They can be uncomfortable in hot weather. You should also be aware that Senny spares are extortionate (lead and ear pads around £50

). Cords on Sennys always break near the plugs (don't forget the 2 year warranty). I cut the brocken cable and glue a little socket to the plug, so that I can make my own much more durable cable.
The DT-770s are comfortable all year round. They do not have a replacable cord. If you don't hear the Sennys you will be vey happy with the Beyers.