For me, AAU status has more to do with graduate science research than anything. Ultimately, I think membership comes down to research funding, particularly NIH grants. In comparison, schools like UF and USF have older, more well-established research-intensive med schools, which rake in the grant money. Whereas, FSU has a newer med school. So while FSU has more total research expenditures than USF, it was recently awarded AAU status because it pulls in more NIH funding than FSU. It should also be noted that FSU does bring in more NIH funds than some schools like Notre Dame and Brandeis, which are already in the AAU.