I'm curious as well, having never heard the DT250. I have owned the CD900ST for years, however, and I also once owned the DT531, an open headphone which has been described by some as sounding like a DT250 but airier and with a wider soundstage. Briefly, the differences with the CD900ST and the DT531:
-The DT531 has better extension in the highs.
-The DT531 mids are slightly laid back. By comparison, the CD900ST sounds more forward in the midrange, the lower midrange specifically.
-The CD900ST has more microdetail. In this aspect the CD900ST trounces the DT531, which seemed to have difficulty resolving finer details.
-The CD900ST is a bit more transparent.
-The DT531 has a peak in the highs around 10khz which I sometimes found fatiguing. To me this was the biggest flaw with the DT531. Fortunately those who have heard both have said the 10khz peak is more pronounced on the DT531 than on the DT250.
My preference, albeit not by a huge margin, was for the CD900ST, which I think is one of the best headphones for midrange lovers. Hopefully this will tide you over until someone who has actually heard the DT250 along with the CD900ST can chime in.