1) it doesn't matter
2) the difference in rigor probably isn't that great if you choose your classes and professors wisely
3) the assumption that you automatically will have a better GPA at one school over the other is a fallacy
4) Where do you fit in better? That's a far more important question.
For those that don't deny the importance of superficial factors, if you're a straight male, the female students at MSU tend to be more attractive than those at UMich. At least that's the trend I noticed while living and dating in Chicago after med school. Maybe it's only true of the alumni...