both are kinda overrated imo
But I'll take Rosen out of the two. Darnold is simply too prone to mental errors. His decisions are often ass, and his picks are usually bad picks - such as not seeing the coverage, throwing into double coverage, throwing with bad mechanics off his backfoot, and so on.
I've said it before and I'll say it again. Darnold has a lot more question marks than Goff going into draft, and I thought Goff had tons of question marks to the point I wouldn't pick him in the first round.
And neither compares to Ryan, really. Rosen is more of a deep/intermediate passer but his pocket presence is far off from Ryan's. Darnold's decision making and reading of defense is far worse.
Of course, Ryan is a NFL vet, so it's kinda silly to compare him to these guys anyway. style wise they have nothing in common tho.