Congratulations! You have two awesome options. You cannot go wrong here. Academically, Michigan offers the PPE major (similar to Wesleyan’s CSS major), and while classes will be larger in some instances, in others, there won’t be much of a difference. That being said, LACs definitely have the edge where class size and interaction with faculty is concerned. Michigan will offer more academic options and more breadth and depth in your major. Neither will prepare you better for graduate school. Only you can determine your graduate school readiness.
If cost is not a concern (and Michigan is more than $6k cheaper than Wesleyan the first year…more like $10k cheaper when you factor in cost of living), I would go for fit.