Tonali reportedly doesn't want to play as regista, he prefers to play higher, as B2B. Generally, I think he is a top talent which can develop into one of greatest central midfielders in the world, but his current skills are a bit overhyped. He has still plenty of bad matches where he makes a lot of mistakes. In my opinion he needs a lot of work to be better player than Brozović or Barella (ofc, he has a lot of time to achieve that) and for this moment I would take Kulusevski if I have a choice. For this moment we need a Sensi's replacement, who can give us new options in attacking phase.
But generally I would be really glad if we get Kulusevski now and Tonali in the summer. Brozović, Barella, Sensi, Tonali and Kulusevski is great set. It would be ok then for me to have also Gagliardini and Vecino as bench players who can add some strength and centimeters.