And I think something like only 2% rejected from their first choice are accepted into their second choice (I honestly forget where I saw that statistic though, so take it with a grain of salt).
I would spend some serious time figuring out your first choice school and making sure your essays demonstrate a passion for that school. Keep in mind, in terms of cross-listed majors (ie. biology), the major itself won't be all that different. What will be different are the other graduation requirements. Speaking as a CAS alum, CAS offers a traditional liberal arts education so you'll definitely want to buy into taking courses from a variety of disciplines (which I think is fantastic).