You may regard the 1946 version as a classic because of the Bogart-Bacall pairing. As a literary adaptation, this version, however, is much better.
First of all, the plot stays true to the novel, whereas the older version had a plot ruined by the restrictions of the Hayes code, so that it contains numerous loose ends and unexplained developments.
Secondly, Robert Mitchum impersonates Marlowe much better that Humphrey Bogart. Bogart essentially recycles his role of Sam Spade in "The Maltese Falcon". Yet, Spade and Marlowe are very different characters. While Spade is a cynic who just barely remembers the remnants of morality (and Bogart is brilliant in that role), Marlowe is way beyond that point. He walks around people in a distanced, almost detached way. Only when he spots a glimpse of humanity in his fellow men, he is willing to engage himself (as with General Sternwood in "The Big Sleep"). Mitchum plays this character with great understatement, as it should be done, while Bogart makes Marlowe just another hard-boiled detective, which could be replaced by any other one.
Finally, both Sarah Miles and Candy Clark (while not being necessarily great actresses) bring over the lunacy of the Sternwood daughters beautifully. While the scenes between Bacall and Bogart a great, they are out of place in this plot, in which there is no place left for romance. It might have been appropriate for the characters of Marlowe and Linda Loring in "The Long Goodbye", but hardly in a movie adaption of a novel, in which Marlowe remarks "both Sternwood women were giving him hell".
So, while this movie transfers the plot to another time and another place, it is a much better adaption of the novel than the version often regarded as a classic.
