Arsenal will win, they have been playing great lately. United depends to much on key players and those players have been failing lately, Rooney and RVP are out of form.
I think United will win or maybe draw but probably controversially. Though United do have the ability to win fair and square. They'll have to play 10/10 to win this one, I'd be surprised if they win playing anything less than a 9 performance.
I'm going with an Arsenal win here. An away win in Dortmund will do wonders for them. They are high in confidence, and what better time to beat United then now?