I’d never call UCLA preppy. It’s far too diverse, in all ways. And BC does have its brand and incredibly strong college loyalty. I went to UCLA grad school and now live in the greater Boston area.
OP is looking at a college exchange year, not an admit. Summers in New England are close to awful. No one should envy them, unless you’re about sailing or live in the mountains. The humidy is killer. Personally, I feel the “great college town” is more about the presence of Harvard and MIT, other schools. Not necesarily a better lifestyle, for students. Opinions willl vary on that point. Of course.
So, OP, you’re going to get differing opinions but it boils down to 3 great US choices.