The DT990 will give you a slightly more extended and airy-er top-end (Treble) due to the open design.
Its been too long since I've heard a DT990-250Ohm but when I compare my DT770-250 to my 990-600, the DT990 has the treble properties I mentioned above + it gains a lot more Mid-Bass accentuation than the DT770. I find the DT770s to extend slightly better in the bass.
The 600Ohm DT990 also is slightly faster sounding but that may be due to the extra extension you gain up top.
The DT990-600Ohm's Treble is the best I've heard in any Dynamic, regardless of the price.
I would name and compare it to Flagship Dynamics but I'm sure I'll get flamed by sensitive owners who seem to hoard Head-Fi.