I’ll jump in on the simpler first question first. CAS stands for College of Arts and Sciences. This is a more common designation for what you have described as Berkeley’s College of Letters and Sciences. Wesleyan does not have an academic division along these lines, as the entire university is essentially a college of arts (letters) and sciences.
Regarding your second question, the case for Wesleyan has been fairly well made on this. Berkeley may be preferred by some students for several reasons. One may be the greater social options available at a university. Another may simply be the feel that some students experience while on the campus.
However, you have stated that you liked Wesleyan’s feel. I’d recommend you use caution in overriding this positive experience through over-analysis.