Use this: the selectivity ranking based purely on objective factors is basically twice higher at Amherst than at UMich.
http://talk.collegeconfidential.com/college-search-selection/1770201-selectivity-ranking-national-us-lacs-combined.html
Using the “Little Ivies” history is also a good selling point (try telling it like this: “Big” Ivies have many students, “Little Ivies” have fewer students, but all will attend the same top graduate schools. Those are just different environments).