Madrid forward Ronaldo and Barcelona star Messi both are candidates for the 2012 FIFA Ballon d'Or award, as they broke a number of records in La Liga last season.
Ronaldo was also key in Portugal's march to the Euro 2012 semi-finals earlier this summer, but Casillas admits that there is nothing to choose between the duo at the moment.
"We are talking about the two best players in the world now and this always make for a great debate," he told Mexican television station Univision. "Some think Messi, others think Ronado is better. I cannot opt for any.
"If I had to choose one, it would be Cristiano because I know him more for we play together at club level.
"We are talking about the two best players in the world right now, and I think that if there were only one, it wouldn't be as good for football."
Both Ronaldo and Messi will be back in competitive action this weekend when the 2012-13 Primera Division season gets underway.