Firstly I want to point out that I'm a life-long Man Utd fan and have seen them both wear the Utd badge.
Currently I'd say Ronaldo is the better player because he plays consistently well in a higher ranked league and European/Cup matches even though LA Galaxy wouldn't have been so successful without Beckham.
At their peaks I'd go for Beckham because he had more influence on winning trophies than Ronaldo. He'd always get the ball to the strikers to score and bag goals himself from dead ball situations. Ronaldo is more selfish with the ball and has a better striking ability but when Beckham hits the sweet spot, it's pure gold.
If we could have one of them back, Beckham would easily get my vote ;)