The NFL and college is a completely different ballgame. You're correct in that some prefer one or the other, but I have a hard time believing Harbaugh's personality and goals would prefer the college game after being so close to a Super Bowl title (which is something Saban was not even close to achieving with Miami).
Michigan fans are quick to point to Saban as 'proof' as to why someone would make this move, but for the large majority of coaches entrenched in the NFL, it's a demotion regardless of what program they're going to. Recruiting, less practice time, dealing with alumni/donors, etc. is not optimal for many that also have legit options in professional football. I don't care what money Michigan is offering because the NFL can match any dollar amount that a college throws out there. And with most jobs for most people, if you just chase the most money instead of happiness or satisfaction (or fit), you're not going to end up happy or satisfied.