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."
TV low point: Jumping on the couch on the set of Oprah and proclaiming his love for Katie Holmes.
Movie low point: Cocktail (1988), which received only a 14 percent favorable rating from critics on Rotten Tomatoes.
Recent films: Night and Day in 2010 and Valkyrie in 2008.
Breakout role: The entrepreneurial Joel in Risky Business in 1983.
Number of movie franchises involved in: 1 - Mission: Impossible.
Number of movies in each franchise: Four.
Biggest paycheck: $75 million each for M:I II and M:I III.
Highest-grossing film, worldwide box office: War of the Worlds (2005) $592 million.
Highest-grossing film, U.S. box office: War of the Worlds (2005) $234 million.
Best sidekick: Anthony Edwards as Goose in Top Gun.
"Girl with the Dragon Tattoo" connection: The actor Michael Nyqvist (who plays Blomkvist in the original, a role taken over by Daniel Craig in the remake) plays the bad guy Hendricks in Ghost Protocol.
Robert John Downey Jr. Born:
April 4, 1965, in New York City.
Marriages: Two - Susan Downey (current), Deborah Falconer (1992-2004).
Children: One - son, Indio, 17. Wife Susan is pregnant with their first child.
Academy Awards: 0; nominated for best actor for Chaplin in 1992 and best supporting actor for Tropic Thunder in 2008.
Sexy-smart: Included in the 2009 "Sexiest Man Alive" issue, but ranked No. 60 in Time Magazine's 100 Most Influential People in the world in 2008.
Miracle cure: Says studying Wing Chun under fight and martial-arts consultant Sifu Eric Oram got him over his drug addiction.
He said what?!: "I know very little about acting. I'm just an incredibly gifted faker."
Low point: Sent to jail several times on drug charges.
TV low point: Fired from Ally McBeal.
Movie low point: Johnny Be Good (1988), rated zero by Rotten Tomatoes.
Recent films: Due Date and Iron Man 2, both in 2010.
Breakout role: The drug-addled Julian in Less Than Zero in 1987.
Number of movie franchises involved in: Two - Iron Man and Sherlock Holmes.
Number of movies in each franchise: Two in both Iron Man and Sherlock Holmes.
Biggest paycheck: $15 million for Sherlock Holmes: A Game of Shadows.
Highest-grossing film, worldwide box office: Iron Man 2 (2010) $624 million.
Highest-grossing film, U.S. box office: Iron Man 2 (2010) $312 million.
Best sidekick: Jude Law as Watson in the Sherlock Holmes films.
"Girl With the Dragon Tattoo" connection: The actor Noomi Rapace (who plays Lisbeth Salander, a.k.a. the girl of the title in the Swedish original, a role taken over by Rooney Mara in the remake) plays Madam Simza Heron, a fortune-teller, in Sherlock Holmes: A Game of Shadows.
SOURCES: International Movie Database, People magazine, The-Numbers.com, RottenTomatoes.com.