Is Williams College or Harvey Mudd better for a STEM-focused student?

The biggest practical tradeoff is depth and intensity of STEM immersion versus breadth and flexibility. Harvey Mudd is built around science, math, and engineering, with a core curriculum that every student takes and a campus culture heavily centered on technical work. Williams gives you strong STEM in a liberal arts setting, so you get smaller departments and serious research access, but the overall campus is not dominated by engineers and coders in the same way.

For a STEM-focused student, Harvey Mudd usually offers the more concentrated experience. Its curriculum is explicitly designed to give students rigorous quantitative training across disciplines, and because it is part of the Claremont Colleges, you also get access to courses, clubs, and social life beyond a single small campus. That helps with your concern about feeling stuck in one campus culture, although the academic atmosphere at Mudd itself is still unmistakably intense and STEM-heavy.

Williams is the better option if you want strong science and math without having your whole college identity revolve around STEM. It has excellent undergraduate teaching, substantial research opportunities for a liberal arts college, and a broader intellectual and social mix because many students are focused on humanities, social sciences, arts, athletics, and public service. That tends to create a more balanced day-to-day environment for someone who wants to stay deeply involved in STEM while also exploring other fields.

One important distinction is what kind of STEM you mean. Harvey Mudd is especially compelling if engineering, computational math, physics, or a highly technical path is central to your goals. Williams is a very strong place for math and sciences too, but it does not offer the same engineering-centered ecosystem, and its strengths show up more in close faculty interaction and broad undergraduate education than in pure technical specialization.

My read is that Harvey Mudd is better for the student who wants STEM to be the center of college life, while Williams is better for the student who wants top-tier STEM within a fuller liberal arts experience. Since you specifically mentioned not wanting to feel boxed into one type of culture, Williams sounds slightly closer to what you described, unless you already know you want a very intense, tech-focused environment.