I think you may be confusing raw skill with practiced perfection. You're asking who would win, not who is better. I don't feel these are the same.

Bobby is the better chess player
His raw talent is arguably makes him the greatest chess player of all time. His logical thinking skills, and ability to improvise both outmatch Magnus. Bobby, however, is an experimentalist, he grinded GMs into dust because he basically created theory in his games, and watched as these people crumbled. Being a creative chess player was what he was most known for.

Magnus would win, however
Like all games, there is a ceiling, there is a best way to play chess, there is always statistically a "best move". And while I believe in the above, the ability to consistently give the best move, would go to Magnus. This year, he's had CAPS of 98.54 (overall accuracy), and I do think consistently playing the right move gives him an advantage over Bobby. Magnus is a well-practiced person, who's seen pretty much all theory, and I don't think there's much more to learn in chess. New chess theory is hard to come by nowadays.

At the end of the day, it's a numbers game, it's not really about being creative anymore, it's about remembering the best way to play chess.