this is honestly a no-brainer decision. it seems like he has a good shot of getting drafted in one of the first 2 rounds of the nfl, securing a pretty decent rookie contract, comparable if not more to the signing bonus he'd get as an MLB player after he signs his rookie contract
even as a backup on an NFL roster he'd have much more national exposure than he would as a minor league player for years, and i can only imagine that endorsement opportunities to make money outside of his professional contract would be much more feasible as an NFL player
even if he doesn't make it as a star or a starter at the NFL level, he can still make himself a very good living as a backup QB throughout his career.
i will admit i do not know much about his MLB prospects, but the only way deciding to go the MLB route trumps any of this is if he makes it as a star player in the MLB, and this is something that is far from guaranteed despite his high draft position in the MLB draft, and is a process that would take roughly 3 years before he'd even be at a level where he would be considered as a call-up to the ML level
there is no decision here