Thats a hard debate and it could be debated from both sides forever. Let me start by saying I am a huge Messi fan and I hate Ronaldo. Ok, I don't hate him, but his diving, whining, and selfishness with the ball are three reasons among many why I wouldn't want him on my team.
But with that being said, you simply can't overlook what the guy has done in the past two years. CL winners, EPL winners, Carling Cup winners, Club World Cup winners, etc etc. Ronaldo won the golden ball (most presigeous award in europe) among many other personal accolades.
Leo Messi on the other hand, didn't win anything last year. RM won La Liga. Barca was knocked out of CL by Man U. I don't even think Messi scored the most goals in La Liga (but I could be wrong). BUT - to me, a lot of that stuff is besides the point. CR7 fans always say "well Messi didn't win anything" and of course thats true, but neither did countless other STARS. When Messi is on the field, he is the most exciting player to watch no matter who Barca is playing. His dribbling, passing, finishing, are all flawless. When Messi is off the field, Barca's attack looks lost. When Ronaldo is off the field, Man U still looks fine. Messi is ESSENTIAL to a Barca team which many people think is currently the best team in the world.
And to top it off, I have never seen the guy take a blatant dive. He gets fouled more then anyone I have ever seen, and he tries his best to stay on his feet at all times.
Ronaldo is a NOTORIOUS diver. Ronaldo has no loyalty to his club as he showed last summer. Ronaldo is arrogant, cocky, etc. These are all things which give him a huge black eye IMO and while I recognize he is probably the best player in the world ON THE FIELD, I think you have to factor shit like this in when making a decision.
Did I mention Leo Messi is 21 years old????
MESSI = Best in the world