against the alapine d5 or Nf6?

against the alapin two moves are possible: d5 or Nf6 I know both are correct but which one is better?

I use Nf6

More interesting to play

I always played ...Nf6 as well as against the Morra.

Look at the Qd5 line where black plays Bf5 instead of g4
I have had great results with this, and most white players do not know the theory

For what it's worth Evgeny Sveshnikov, a leading expert in the Alapin variation, considers 2...Nf6! to be the best answer. See for exemple his 2010 book "The complete c3 Sicilian".

Fortunately, it comes with its own exclam.

I play g6 and break the center phalanx with d5 if/when white plays cd4.

Ideally, none of them is better. Modern chess engine evaluate it as 0.0. It depends which you can play better.

I myself like to play d5 against a "normal" Alapin, although I can imagine that this line is definetely not for everyone!
Against the delayed Alapin I play Nf6, because I don't want to waste a tempo playing d5 when I've already played d6 (I usually play the Najdorf, so d6 is my second move).