Evans has one great NBA skill right now: rebounding - just like Millsap had when he came in.
And that's all the Jazz coaches are asking Jeremy to do, just get to your spots on offense and hit the boards hard. Any passes he gets are going to be going to the hoop.
Hayward, on the other hand, has a lot of skills, but he needs to improve on all of them: shooting, dribbling, passing, etc. Flash was a very good shooter 2 years ago, then his percentage really dropped. I'm just guessing, but it was probably due to him becoming a "go-to" player, drawing more attention and rushing or forcing up some shots. Hayward is being asked to do more for the Jazz: bring the ball up at times, drive and dish, shoot off curls.
In sum, I think Evans will have the better stats right away, but Hayward will probably follow a Matthews-like curve. He'll get action early and won't impress, but mid-season onwards, he'll have some games with double-digit points, 4-5 rebounds, 4-5 assists and a couple of steals.