I live near Wesleyan and know kids who’ve gone there, so I’ll try to fill in the gap. I’ve been to Northwestern but don’t really have any other first hand knowledge of it otherwise. I have a daughter and a niece who went to BC and have nothing but good things to say about it. But I’ll focus on Wesleyan since others have spoken about BC.
Science is one of the things that Wesleyan does really well. Research opportunities are not just available. Students WILL be engaged in research with a faculty mentor. Although Wesleyan is a small LA college, it’s on the larger side for these schools (3000 undergrads) AND it has a graduate school in science. A student can spend 5 years there and graduate with a BA/MA in Chemistry. There is also a PhD program in Chemistry. The point is that there is research going on at Wes. On a related note, they have an entire College of the Evironment.
As for student life, there are D III sports at Wesleyan, the football field is a hub around which the rest of the campus is built, Bill Belichick went here, and they have great athletic facilities. But there is not a rah rah sports culture here. They certainly have intramural and club sports.
The student body is definitely liberal. The school is not in or near a big city although Middletown has a great restaurant scene and some live music. Hartford isn’t far and big music acts play there. New Haven is also easily accessible. It’s half way between NYC and Boston, so they are accessible for day trips.
Have you visited the 3 schools? Admissions at any of the 3 will probably make faculty and students available to answer questions and just tell you about the place.