I say Teddy has more immediate upside as he has years of experience over Darnold. The difference between the two is very evident when you see them on the field. It's not close. Teddy has 4 years in the NFL to 0 for Darnold. 3 years college experience to Darnold's 2. 4 years High School at one of the best area for NCAA and NFL talent to 4 games for Darnold plus 7-8 years of pee wee football. Numerous high profile QB camps. He can read NFL defenses, audible, look off defenders. All things Sam has yet to learn.
Teddy has never been a running QB. He has always been a pocket passer. Sam is more likely to take off and run.
Teddy has a much quicker release , is more accurate, especially deep.
Darnold has struggled with the deep ball in practice, lots of under thrown balls. This has continued through this week. That's probably why Bates has not had him pushing the ball down field in the first 2 games.
I don't think Teddy has had the opportunity to show his ceiling. He played in an ultra conservative system so at this point, both Darnold and Teddy's ceilings are unknown.