I always pick Divination as one of my opposed schools. A majority, if not all of that school's spells could be cast "tomorrow" after you've had a chance to rest and study so there's no real reason to have them memorized on a regular basis. In that case having to double up on spell slots won't have that much of an impact. As far as the other school, I'd have to say personal preference, they all have one or two iconic spells. Oh, and as someone mentioned, scrolls.
It just depends on your role, personality, and character concept.
As a side note, if I were DMing a group I wouldn't allow Universal to be an opposition school. That's what, 5 spells? Not enough to count.
Have fun.