What's telling for me is that the coaching staff chose to have Jackson run the ball in critical situations and not Spiller. Whether they needed a first down, near the goal line, pass protection on 3rd down, etc...
Spiller is an awful blocker, which severely hinders his playing time and trustworthiness.
His vision leaves a lot to be desired as well. He just doesn't see the holes and cutback lanes the way Jackson does.
Jackson is still the most reliable player on the Bills offense. I don't see that changing this year just because he's 365 days older.