I think Mariotta is overated. Plays in a great system...as Chip Kelly has shown in the NFL making Mark Sanchez and Nick Foles pretty decent QBs compared to their past. What QB hasn't put up great numbers in that system? If Mariotta can play for Chip Kelly, worth every bit of the first pick...unfortunately, that's not how it works in the NFL. NFL coaches, outside of Jim Harbaugh/Pete Carroll/Chip Kelly, have proven they are unwilling to adapt their system to the QBs strengths...hence the premium on guys like Peyton Manning, Aaron Rodgers, Tom Brady. Guys who are rare. Mariotta is going to get ruined like RG3 was.
I think Winston will be a fairly good QB. Not in Andrew Luck's level (he's a once in a generation talent), but somewhere in the second tier of QBs. He makes some amazing throws, and just has a natural feel for the QB position...scouts widely agree on this. His problem is whether franchise teams can trust him once he leaves the football complex.
I think Winston will end having the better NFL career of the two.