Sigh. Getting a first down was better because it would've iced the game, scoring meant the Niners got the ball back so definitely not "better".
Your whole view on this is wrong imo - the award is literally called "Most Valuable Player", not "best performance in the Super Bowl". RBs as a whole are simply not valuable at all, proven by the fact that the two featured RBs in the game went undrafted. You could've probably put 30 to 50 RBs in the game for the Chiefs and they would've done the same as Williams, who isn't even that good of a RB. It's all about Reid's system in which RBs historically play well. Same goes for Mostert and the Niners run offense.
With Mahomes, you couldn't find more than 3 to 5 other players on earth who could do what he does (not even 1 if you wanna stretch is). He's the reason their offense ticks, he's the reason they can explode at any point, and he was the reason they scored 21 points in Q4. It's totally justifiable giving it to Mahomes, and not to Williams because he had a nice stat line. If you really want to have a skill player win it other than the QB, Hill had a bigger influence on the game than Williams.