Black's 6 move in open Sicilian Is moving the Knight to c6 considered a stronger move than the Najdorf pawn to a6?
You probably mean Najdorf vs Classical variation in Sicilian defense:

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1. e4 c5 2. Nf3 d6 3. d4 cxd4 4. Nxd4 Nf6 5. Nc3 a6 (5... Nc6)


Since they're both normal opening lines, you can't say which one is better. This is mostly matter of taste.

On the other hand on the highest level Najdorf is more played (I can't even remember a top game in the Classical variation). That means Najdorf is more trendy now.

It's true that Classical variation is not so attractive any more, because it is deeply analyzed. Najdorf has still a lot to offer...