I've watched both Rosen and Darnold so far. Just some general, very preliminary observations...
Darnold - physically, his arm is average. His throwing motion has a hitch which causes him to lose velocity, the spiral and telegraph his passes. It's a motion similar to Blake Bortles. Made some bad throws that showed his inexperience, which is to be expected. And Tee Martin's offense looks like a hybrid pro-style, and college read option, which would slow his development a bit. So that's the bad. Here's the good - He has superb pocket awareness, and has been well coached there. He slides in the pocket effortlessly and throws really well on the run. His accuracy is also very good to excellent when he is on rhythm. Throws with excellent anticipation at times - he threw some receivers open and put the ball into some tight NFL windows, which was impressive. Seems like a good kid. Based off of what I've seen yesterday, he seems more like Chad Pennington to me based on his physical skills (which is great, loved Chad), then say the next Andrew Luck. My biggest concern with him thus far are the mechanics of this throwing motion and arm strength.
Rosen - above average to very good arm strength. Superb throwing mechanics and looks the part back there. Good mobility and showed some leadership last week in his come from behind win. Again, the offense looks like a hybrid, which will slow his development somewhat, but he is by all accounts very intelligent and should pick up an NFL offense. Very talented player. Need to see more games to better evaluate him because he was in hurry up mode all game.