I don’t think there’s a very clear answer. They’re the consensus top 2 in the league, similar age, career achievements and late bloomers. Both relatively injury free so far though Jokic is more available, both seem intent on staying all or most of their careers with their draft team.
Personally, I think it’s harder to build a team around Giannis you’re going to need people to compliment his weaknesses in shooting, free throws and late-game situations. Jokic dosen’t need as specific tailoring of teammates to his skills so your team might be more consistent through roster transitions. I guess you could knock Joker down for his defense but finding a power forward like Gordon who can cover up a bit on defense is easier than finding multiple good shooters.
Secondly, Joker’s game atleast appears like it’ll age better because it’s more IQ and finesse than athleticism. That might change if Giannis develops his game with age like Lebron did but it’s a less certain bet than Jokic.
Jokic is deceptively athletic but other than baseline roll drives there’s nothing he does right now that he shouldn’t be able to do at 36. You’re really talking about conditioning, not athleticism. Jokic has superb conditioning and there’s no real reason to believe it will get worse at a higher rate for him than anyone else.