Prolific is one of the best words to describe Cristiano Ronaldo and Lionel Messi. They both have easily outscored the rest of the world's best players. Two consecutive seasons of over 50 goals for both players. In 2012, we saw record-breaking performances from both players: Messi becoming Barcelona's all-time leading scorer, Ronaldo scoring 100 goals faster than any Madrid player ever.
It's a natural inclination to want to differentiate the two players based on the goals they've scored because of how prolific they are. It's a simple statistic that many feel sheds light into who the better player is.
This season, Messi holds a strong edge in goals scored, having netted 68 across 49 appearances for a ratio of 1.39. By comparison, Ronaldo has 53 goals in 46 competitions for a ratio of 1.15. Both incredible, but Messi's better..right? Well...
The problem with the goals scored stat is that it doesn't account for a lot of factors. For one, while Messi's ratio is currently better, it's not better across the previous two seasons, standing at .88 compared to Ronaldo's .90 ratio.
One might be argue that Messi's goal scoring rate is higher than Ronaldo's over their respective careers, but there are two driving factors for this.
The first is that Ronaldo has played his entire career on the wing, spending many years acting as a true winger (one who brings the ball forward and delivers a cross or pass to the striker). He wasn't meant to be the biggest goalscorer.
In contrast, Messi plays centrally and in the years he played wide, he had to cut into the center. That's not Messi's fault, though, as Barcelona just hasn't been great aerially the past few years. They've also had problems keeping strikers (just ask Ibrahimovic and Eto'o who are both world class), so Messi has had to assert himself as a principal scorer.
The second and more important factor is that they play for different teams. Ronaldo's played in three different leagues for three different teams across his career. He's had to adjust to new teammates, new offenses and new playing styles.
At United, he had no real playmakers to set him up. The midfield was hardly the most creative in the world, and the style of the Premier League is rougher and more aggressive.
Looking at Messi, he's been at one club his whole life, which is admirable. He's gotten the benefit of years of play with the same teammates. He has arguably the best midfield in the world behind him and has for years. He should be expected to score more.