SQ? They're good, I passed up on some far sexier and more exotic phones for them, because of their quality and versatility. Yes, I'm biased, as I've heard a range of Senns and don't like the sound, so that's my disclosure, take it as you see fit.
I don't have the dB specs, but it's safe to say the Senn will have higher isolation - it's a DJ based phone. That also means bass emphasis, which reduces clarity of the other freq's, and overall SQ.
If isolation is your priority, it will provide noticeably more than the DT250. It depends how much you think you need.I listen to mine at home, so massive isolation is not a priority, but they provide significantly more than my open phones, but not as much as canals.
Bias aside, the DT250 is far more neutral and therefore will suit a wider range of music; if the music has bass, you'll get it, it not, you won't. It imposes far less of its own sound 'style' on music. It's a music production phone that is great for listening too, which not all are.