The mistakes Tua made last year (when he was in the game) were mostly from youth and lack of game experience. Unfortunately, the mistakes Jalen made were more developmental (or lack thereof) in nature. Which part of developmental is coaching, which isn't completely his fault. It's not a knock on Jalen. It's just a reality. One of the questions Saban may have to ultimately ask himself and his coaches (when deciding on a QB) is, "Whose mistakes are more fixable?"
I go back to a discussion we had shortly after the season. Can Jalen realistically fix not being able to process defenses/information at the speed they are coming at him? Because the speed of the defense doesn't hardly make Tua flinch. He reads the defense and seemingly in a millisecond the ball is out, and out to the right receiver. Can Jalen realistically improve his field vision? Because as we've seen, Tua's field vision is nothing short of remarkable. He sees the entire field. All WR's are in play, no matter where he's looking at the time.
As of right now, IMO, any deficiencies in Tua's game are very, very fixable. They will correct themselves through more reps and experience. Jalen's areas of weakness are more complex in nature and I'm not sure are as easily fixable as Tua's.