It seems so clear to me. You go with Taysom - no brainer. I am giddy about Mangum and his future, but you are likely never ever going to have a QB with Taysom athleticism, leadership, and experience.
Taysom would have made Michigan pay with the defense they were running and his scrambling ability. His passing looked so much approved in his game at Nebraska. Heck he scrambled for a TD and a 30 yard plus run on a broken wheel at Nebraska.
You don't just throw Heisman contenders once in a lifetime players like Taysom to the curb because of Mangum's potential.