It's a bit different coming through Barca's system to Sporting's. One is one of the biggest clubs in the world, the other is known as a stepping stone to bigger things, as to a degree were United. As Messi was the best player in the world already by the age of 19, no amount of money would have bought him for the next 10-15 years. There's also the fact that Messi is clearly the better footballer regardless.