But, lo, the paths of these two very different actors have crossed in unexpected ways. Both, for example, have starred opposite Michelle Monaghan. For Downey, it was in Kiss Kiss Bang Bang in 2005; for Cruise, Mission: Impossible III in 2006.
Both of their new movies tie them to another big-budget picture, The Girl With the Dragon Tattoo, or at least the Swedish original.
And both had supporting roles in the 2008 R-rated comedy Tropic Thunder, with Downey creating controversy by playing an Australian actor who thinks he can pull off playing a black man in black face (a role that earned him one of his two Oscar nominations), while Cruise took a small and surprising cameo that played against his image - as a bald, potbellied movie producer.
Here, for your perusal, is a comparison of the two actors. How do they stack up? You decide.
Thomas Cruise Mapother IV
Born: July 3, 1962, in Syracuse.
Marriages: Three - to Katie Holmes (current), Nicole Kidman (1990-2001), and Mimi Rogers (1987-1990).
Children: Three - daughter Suri and adopted children Isabella and Connor.
Academy Awards: 0; nominated for best actor in 1989 for Born on the Fourth of July and in 1996 for Jerry Maguire, and supporting actor in Magnolia in 1999.
If you think he's sexy . . . : Voted by People magazine three times as one of the 50 most beautiful people in the world. Chosen "Sexiest Man Alive" by People in 1990.
Miracle cure: Says he was cured of dyslexia by the Church of Scientology.
He said what?!: To Today show host Matt Lauer in criticizing Brooke Shields' postpartum depression: "I've never agreed with psychiatry, ever. Before I was a Scientologist, I never agreed with psychiatry. Here's the problem. You don't know the history of psychiatry. I do."