This thing about being third choice being a problem is rubbish.
If Barca and Madrid were his preference, it doesn't mean he'd sulk at the thought of coming to United and underperform. First of all, he's always spoken well about the club, so it's clear he has a fondness for the place. Secondly, he'll be playing under one of the best managers in the world, in a squad which, with him included would be genuine title contenders. We don't know everything about his character, but we can be fairly sure that would motivate him.
Also if his ultimate dream is Barcelona or Madrid, it's probably because it's the best chance to win a top league and compete for Champions League titles. Surely, if he ended up here, we could soon offer him those opportunities.
And finally, if it's just a thing he has for either of the Spanish giants and no matter what we do he'll want to leave, his dream move won't come about by underperforming with us. He'd surely want to prove himself with us and make Barca or Madrid come in for him. If this is the case, we'd get 2/3 good seasons from him and more than likely make a profit on him.
If we have any chance of signing him, we should take it.