<p>I stand by my original answer: both Swarthmore and Haverford have excellent biology programs.</p>
<p>There are many reasons why one student might choose Swarthmore over Haverford or vice versa, but the absolute potential to get an excellent undergrad Bio education would not be one of the more significant reasons.</p>
<p>If you are looking for statistical differences, I would suggest financial resources and per student endowment as important measures. Additionally, I would recommend visiting both schools and seeing for yourself. Of course, there is always that other little factor which helps many students decide: which one mails you an acceptance letter! The two schools have somewhat different admissions profiles.</p>