I have both the 250 ohm Pro model and 600 ohm model and I've done a side by side A/B comparison using a DNA Sonett amp which can easily drive both.
The big difference is this: when listening with the 250 ohm, I like to have the volume dial at around 6 or 7; if I turn it up higher, I don't get much improvement in quality of music (as in it doesn't get richer or more detailed, only louder). When listening with the 600 ohm, I can turn the volume dial comfortably up to 8 or 9 and the music becomes noticeably richer and filled with details that I never was able to experience with the 250 ohm.
I haven't done an exhaustive comparison between the two because I immediately realized how much I like the 600 ohm version with my setup but expect one in the near future.