Messi would not instantly make United title favourites - well maybe on paper, but realistically, not. Man Utd have been quite prone to messing it up big time with big players recently, why would Messi be different - he'd certainly push Bruno away from where he is now too, forget what the rest of the attack has been working towards recently - he's worth chancing it of course, but I'd not be confident over Liverpool and City purely to it.... because, unfortunately, Solskjaer.
He's not already well travelled like Ronaldo to Juve.... it's a big risk. At least at City, theres already language and Barca connections, also KdB is very much on his level footballing wise - which Pogba or Bruno are not quite up to.
All in all, I don't know, probably Sancho... likely less risky, I wouldn't want Messi to end up like di Maria did at Old Trafford, theres nothing to really bed him in easily. Mega challenge for him.