I have expressed my opinion before and I reiterate here. Migrating to math SE (or anywhere else) is not ethically correct. We can only decide that the question is bad for MO, not that it is "more appropriate" for another site. As for math SE, from what I heard, it is populated by undergraduate students trying to cheat by posting their homework questions, and by mathematicians helping the cheating. Perhaps I am wrong, but if I am correct, migrating a question there would be even more wrong.
Edit For those who still do not understand why it is wrong: suppose that you submit a paper to Annals of Mathematics, and the editorial board there send it to Mathematische Annalen - without your approval and without even notifying you.