have basically not played competitive football for two full years, while Tanner will be coming off a full season?
Something to keep in mind here: As of September 2016, Taysom Hill will have played in 25 total games. Tanner Mangum will have played (assuming he stays healthy) in 13 total games (assuming BYU makes a bowl game). 19 of Taysom's 25 games were played in 2012 or 2013. He will have only played one half of competitive football in nearly two full calendar years by the time September 2016 rolls around. Tanner will have 13 games under his belt, all within the past year.
You really think Taysom's going to be sharper than Mangum? Mangum's a sharper passer to begin with.