Marcus Mariota was and is a better QB than Jamies Winston. This is shown by, most obviously, the statistics. Winston threw 17 interceptions in his last year playing college ball. Mariota threw all of 2. Mariota's comp. rate was 67.2%. Winston's was 3% lower. Another notable difference is the number of touchdowns. Winston had 24 touchdowns. Mariota had 38, much more that Winston. Under pressure and with blitz, Mariota outperforms Winston. Mariota is also faster and can run. Winston, not so much. Mariota beats Winston in statistics across the board.
Winston was charged (probably guilty) for assault. Additionally, he was arrested for shoplifting crab and other seafood. Mariota had no such problems. Mariota was never kicked out of practice for being an idiot. Jameis was.