There is no question at all that Manning is more valuable to his team than Peterson is. Manning is a coach on the field, gets the very best out of his teammates, and has led his team to a 13 win season, a bye week in the playoffs, and very possibly to a spot in the superbowl.
Peterson is the feel good story, having an outstanding season when his health was very much in doubt. He's not a guy who lifts players around him, and whether they even make the playoffs is up in the air.
This isn't a tough decision at all. Comeback player? Peterson could certainly win that.