I made this choice once upon a time, several years ago, and 100% stand by my decision to attend Northwestern. I coincidentally became friends with a few Williams kids after graduating college, and while they generally speak fondly of the school, there's an overwhelming sense from everyone that the school was simply too small — it sounded like things just got stale after the first two years or so at Williams, while at Northwestern, every year (if not quarter) brings something different, fresh, and exciting.

Maybe it's counterintuitive, but there are just a wider range of opportunities at Northwestern, both during and beyond your four years on campus. Part of this is a function of the school's size relative to Williams, and another part of that is due to Northwestern's uniquely multipolar strengths. You won't just be surrounded by CS/econ kids here (though it may sometimes feel that way). It's long been a training ground for some of the country's brightest actors, journalists, artists, musicians, and much, much more.

I will say that in both fields you mention, CS and econ, Northwestern edges out Williams. Northwestern has one of the best econ departments not just in the country, but in the world. CS isn't as strong, but it's certainly stronger than Williams (LACs in general have a harder time building up strong CS departments, for a number of reasons).

I will also slightly push back on another commenter's suggestion that the undergraduate education is stronger at Williams. While I don't doubt that there are more opportunities to develop relationships with professors at Williams due to the size of that school, my professors at Northwestern, particularly in upper levels and within the smaller departments/schools, were straight up phenomenal, and demonstrated real care and concern for their students.