Is moving the Knight to c6 considered a stronger move than the Najdorf pawn to a6?
a6 is the Najdorf which is most commonly played and Nc6 is the Richter Rauzer which is not as popular like the Najdorf .
With experience I have seen Players below 2000 ELo level do not play the Richter Rauzer variation and they are unfamiliar with this line . So if you are playing some one below that level you can give a jolt if you play the Richter line .
The common strategy for both lines is that Black will do the minority attack on the Queen side and try playing a d5 break in the Centre in most cases .
There is nothing technically stronger as which one to choose b/w a6 & Nc6 but it depends upon your opponents knowledge and preparation .
"Is moving the Knight to c6 considered a stronger move than the Najdorf pawn to a6"
You are talking about 1. e4 c5 2. Nf3 d6 3. d4 cxd4 4. Nxd4 Nf6 5. Nc3 Nc6, instead of 5. ... a6. ...?
If so , as stated, it is a matter of preference and style.
"It's true that Classical variation is not so attractive any more, because it is deeply analyzed."
This might be true to a professional, but hardly true to the general chess-playing public patzer you meet in OTB tourneys.
You probably mean Najdorf vs Classical variation in Sicilian defense:
[FEN ""] 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...