I'm not in favor of either QB btw. I think they can both be great, and most likely will.
One has ARod the other has Peyton potential. Not saying they're either, but what they both bring and the situation that they both fell into can be similar to their comparisons.
I think Mariota landed in a better spot in total. That team's coaching staff is Hall of Fame level. Whisenhunt definitely can turn him into a reliable/top-tier QB under center because he's an offensive guru that's worked with Super Bowl quarterbacks as well elite QB's. Combine that with Mariota's athleticism and that's a great situation for him.
I think Jameis is a better QB though. However like Peyton, he's going to have to teach himself more than Mariota. There is nobody than can mentor him offensively on Whisenhunt's level in Tampa. Tampa's OC's specialty is the spread offense, just doesn't mesh well. Lovie can be like Dungy and build him a better defense. Just build him a mean offensive line and I think dude can be GOAT-level: Peyton Manning with a liver arm and slightly more mobility. Dude seems smart enough to craft his own offense and be the OC + QB.
Given this information, objectively Mariota has a Hall of Fame level staff around him. Just remember that if Jameis' team isn't as successful at the start (my honest evaluation of what's most likely going to happen).
I can see both of them winning Super Bowls climbing their currents routes though. No I'm not trying to predict anything off of just what they've shown, just taking into account what their supporting casts (like coaching staff) offer them in combination with their skill set.