This is what Roger Ebert had to say on his blog today about the relative merits of the two movies:
Ebert: (...) And speaking of Oscars, "The Dark Night" was so much a better film than "Benjamin Button."I may have had differences of opinion with Ebert in the past (notably for The Ghost and the Darkness and My Cousin Vinny) but I have always had immense respect for what he had to say. (Does that make sense?)
In this case, I am inclined to believe him even without watching TCCOBB.
By the way, here are Ebert's reviews of the three movies (highest rating - 4 stars):
The Dark Knight (4 stars)
The Curious Case of Benjamin Button (2.5 stars)
Slumdog Millionaire (4 stars)