I think the regular season Booker has been better. But 2 of the 3 playoff years we've seen Mitchell step up and do something Booker never has. We also see Mitchell play defense at times in the playoffs. Maybe Booker would as well but he hasn't. I don't think swapping Mitchell and Gobert makes much of a difference for either team in the playoffs.But KAT and Gobert are 2 completely different players. It's an apples to oranges comparison.
Booker and Mitchell are basically the same player now that Mitchell doesnt try on D. Mitchell is still better on D than Booker, but Booker is likely the better offensive player (MItchell could take a leap this year and surpass him, sure).