Here is my take on it, Michigan covers easily. The reason is simple...defense.
Michigan defense thrives playing against spread offenses. They chew them up and spit them out. If Michigan score 24, which they will, they cover the spread.
Michigan defense points against the spread since Down Brown arrived:
14 - Cincinnati
3 - Florida (offense only scored 3)
20 - Ohio State (double overtime, 1 defensive TD, an interception returned to the Michigan 13 leading to another TD)
10 - Indiana
3 - Maryland
8 - Illinois (4th quarter TD against 3rd string)
0 - Rutgers
10 - Penn State (yes, that Penn State)
28 Colorado - The one anomaly but proved to be a very good offense.
14 - UCF
3 - Hawaii
10.2 is the average offensive points per game the Michigan defense gives up against the spread.
Michigan is averaging 32.6 points per game with the offense struggling.
I'm no rocket surgeon, but this one seems pretty easy to me. Purdue may have some confidence, but how quickly does that confidence go away now that they are playing the first good defense on their schedule? What does a few three and outs do to their psyche?
The question you have to ask yourself, do you really think Purdue has gotten that much better from last years team? The same team that lost to Wisconsin 49-20 which would be the closest comparison to this game.
Anything under 17 is a steal in this game. It's the one game this weekend that the books screwed up and it's because of a perfect storm of Purdue looking very good against terrible teams (sans an average Louisville team) and Michigan not blowing out some decent teams.
Good luck if you're backing Purdue, but I don't see how this game is close. Just remember, this simple phrase "Michigan's defense owns the spread offense."