As far as I know Napoli are currently not only aiming at Jack Grealish, Napoli are currently focused on the two English League stars like Jadon Sancho (Man United) and as you said Jack Grealish (Man City).
Nevertheless Napoli management still considers and is not rash to make the decision to carry out the transfer market this summer, the Italian League, considering that the two between Sancho & Grealish can bring different risks and potentials, one of which is Grealish entering the age of 29 years and salary, From my observation it seems that Napoli management prioritizes Sancho, for that we will see who is included in the Napoli choice in the transfer market this summer.
Chelsea was also planning to keep Sancho with a permanent deal. But they couldn't agree on the fee yet I guess. Sancho is still considered young so he can also be a good choice for Napoli.

Which one would I prefer? Sancho truthfully as I find Grealish overrated.

There are 4 teams interested in signing Jack Grealish. Besides Napoli, there are Leverkusen, Everton, and Newcastle United. According to transfermarkt, Everton has the biggest chance. But if Jack Grealish wants a better career and wins another trophy, joining Napoli looks the best choice. Yes, he will reunited with De Bruyne if he moves to Napoli.
Reference: https://www.transfermarkt.com/jack-grealish/profil/spieler/203460Everton has the biggest chance? Do they offer him more money? Then yeah, I can understand. Otherwise I doubt it. Napoli, Newcastle and Leverkusen are better options. But it depends on the potential playing time for him at the same time...