This was put to a vote by people on this very same forum about who's forehand is better and it wasn't even close with Alcaraz chosen as clearly having the better forehand.
He routinely hit's more forehand winners than Sinner does in their matches against each other. His top end forehand speed is higher than Sinners. Sinner might hit his forehand at an average speed higher because he just hits the same forehand shot over and over with less variety where Carlos mixes it up more, but firepower for firepower Carlos's forehand is more explosive as it connects with a straight arm, and he can hit better angles with it. Sinner connects with a bend arm and can't generate as much power with that technique.
Sinner however does have a slightly better backhand.
Here is a really good video analysis breakdown of what makes his forehand so good :