Sir Alex Ferguson left Manchester United in 2013 after 27 years as manager
It's a debate which will rumble on for years.
But Manchester United legend Sir Alex Ferguson is standing firm on who he thinks is better.
His most prolific season was the 2007/8 campaign, during which he bagged 42 goals in 49 appearances in all competitions.
And the pair enjoyed a trophy-laden spell together.
Ronaldo won three Premier League titles, an FA Cup, two League Cups and the 2008 Champions League at United, before leaving for Madrid for a then world record £80m in 2009.
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