First off, I'm at the point where I believe one of those two QBs will be the #1 pick in the draft. Anyone else would be an upset. I have Rosen ahead by a slight margin, I'll go into a quick bullet point why:
*Both show a refined passing motion and accuracy that improves from year to year. They both have adequate arm strength, even if neither are freaks like Mahomes.
* Rosen's improvisation maneuvering in the pocket has become underrated around RCF. He doesn't rush for positive yards, but he buys time. That's what you look for in an NFL QB.
* Mayfield isn't just running to manipulate the pocket and coverage. He leaves open throws on the field in order to try rushing for a few yards. He doesn't slide consistently, taking unnecessary hits. There's a line between escaping a rush and being reckless, and an NFL sized front seven would have punished him for a few questionable plays he tries to make.
* Hand size has been a question with Mayfield. I won't write anything else on it until he gets measured. SI wrote about that issue when he lost his job to Davis Webb and then Mahomes at A&M. In bad weather, hand size matters.
* Running a dropback offense for three years, Rosen has a picture perfect base dropping back and setting his feet to throw. Mayfield runs a lot of plays from the gun and with pistol action. While the NFL has adopted more pistol, dropping back from center is still a necessity, and one A&M didn't emphasize as much as UCLA.