I'm not a fan of either of them, but my money is on the guy who has been actively training, competing, is 13 years younger. Elon is rumored to be 70lbs heavier, and you can bet it isn't muscle.
If it was actually muscle--and I'm definitely not saying that it is--it might be close to a fair fight. Weight classes exist for a reason, and that's because mass and raw strength can counter a lot of technique. No one in their right mind would legitimately think that a pre-boxing Julius Hafthor Bjornson (e.g., back when he was the world strongest man and weight something like 450#) would be beaten by a world-champion flyweight MMA fighter.
...But I highly doubt that the 70# or so is muscle; my bet would be that Musk has less muscle mass than Zuckerberg. Still, being heavy and flabby means that if he flopped on top of Zuck, he might be able to "pin" him by virtue of Zuck not being able to move underneath him.