FORM. After scoring 31 goals in the Premier League in 2007-08 and eight goals in the Champions League -- both season highs -- Ronaldo has "slumped" to totals of 18 goals in the EPL and four in the Champions League this season. His goals have come at key times, though. He called the goal that gave United a 1-0 win at Porto in the quarterfinals -- a blast from 40 yards -- the "best goal I've ever scored."
Messi has scored career highs in both La Liga (23 goals) and the Champions League (eight goals), but he was largely shut down by Chelsea in the semifinals -- until the last minute of the second leg when he laid the ball for Andres Iniesta to score the goal that gave Barcelona the series victory on the away-goal rule.
Messi blamed his poor performance against Chelsea on the Blues' overly defensive tactics, but Manchester United manager Alex Ferguson says Messi can be shut down again.
"Yes, Messi is a fantastic footballer," says the United manager. "We played him twice last season, so we know how powerful he can be, but we have the players who can cope with him, although you have to respect someone like that. When my defenders are at the top of their game, there is no one to match them."
STYLE. It's impossible to typecast Ronaldo or Messi. Both play forward roles, but neither is a traditional striker. At 6-foot-1, Ronaldo has five inches on Messi and is excellent in the air and dangerous on free kicks.
Both are excellent dribblers. Ronaldo's game is something he has worked on for hours in training. Messi is more of an artist -- his dribbling is more natural and unpredictable.
Messi's goal against Spanish club Getafe in last season's Copa del Rey in which he beat five defenders has been compared with the memorable goal Diego Maradona, now Messi's coach on the Argentine national team, scored against England at the 1986 World Cup.
DEBATE. Barcelona midfielder Xavi says there is no comparison.
"All due respect to Ronaldo and all the other great players on the world stage," says the Spaniard, "but Messi is proving that he is better than everyone else. The world can see that he's the boss. I've never seen anything like it. In a game, in the training sessions, never. I wouldn't swap him for any player."
It will be Frenchman Patrice Evra's job to shut down Messi -- just as he did in last year's Champions League series United won.
"Messi is a player who you can block 10 times," Evra said. "But, if he passes you once and scores a goal, everyone says, 'Evra played very badly against Messi.'"
Evra hopes his work will keep the challenger Messi from taking Ronaldo's crown as the world's best player.
"Ronaldo won the [Golden Ball and FIFA Player of the Year awards]," the defender says. "He scored 42 goals last season and won the Champions League. This is important. I can show Ronaldo is the best player in the world if I play well against Messi."
Ferguson declined to take sides.
"They are two fantastic footballers with courage to attack defenders all the time, no matter how many times they are knocked down," he says. "They have the courage to get up and attack again. They both have tremendous qualities and at the end of the day, how can you decide which is better? It's impossible to separate them in that regard."