I think the amplifier you power them with has a lot to do with the differences in sound between high and low impedance drivers.
After a night of letting my 600s run, they are less harsh in the highs. Now the differences between the 250s and the 600s are subtle, but still there. The 600s are more controlled and even, with details resolved a little finer. The 250s are more dynamic, more engaging, with a little more space, but less detailed.
I've been switching back and forth all morning, and every time I switch to the other one I think "oh, I like this better." ha ha. So I guess I no longer have a preference. I'll probably end up keeping the 600, but I'm going to live with both of them a while longer to be sure.
I also tried my other DAC, an Audio-GD NFB-3, for a while. That DAC has a bit less resolution than my V-DAC, and the two headphones sounded much more similar, almost indistinguishable.
So if someone were trying to decide whether the 600-ohm is worth another $100, my answer would be "not unless you have a powerful amplifier and a very detailed DAC."
Really comes down to get 600 Ohm DT880s if your amp has enough output voltage.
I have an OTL vacuum tube amp, it doesn't have a high power rating by any means, but it does output more voltage than most of my SS head amps.