Managers like Fergie are always building for the future and thinking of the long term benifits of the club. When one day he leaves, he will leave United in the best possible shape for the next manager to take over.
Capello only takes over the clubs for the short term. He buys experienced players for big money to win titles with immediate return. Look at the state Capello left Roma in when he left, hugely in debt. Look at the state he left Juve in, even if they would not been relegated the average of that team must of been 30+.
Look at some of his buys for Real Madrid -
Cannavaro - 32/33
Emerson -30/31
Van Nistelrooy - 29/30
How will they help Madrid in say 2 years after Capello is gone.
Capello's work for a team is amazing, however for a club I'd take Ferguson any day.