Christiano Ronaldo should continue with Manchester United. English Premier League is more competitive and a player of his stature can hone his soccer skill more in such a competitive atmosphere. Spanish League in comparison is a bit dull. He would have better player as colleagues or as opponent in ManU.
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Ronaldo is a great player and one of the signs of a champion is that he can shine no matter where he plays. A true champion can play in the middle of a desert , on top of a mountain, by the sea , on grassland and yet emerge a champion.
So Ronaldo will remain a great player no matter which team he joins.