There is such a big difference between the locations, that I think that might be a deciding factor one way or the other. Size too, though Penn offers ways to break it down.
Both are top notch academically of course.
I lived in Philly with Penn students, who lived off campus. Penn is definitely an urban university.
For nature, you could certainly get to parks or get out of the city.
I visit the Williams area frequently, go to MassMOCA and the Clark and stay in Williamstown. I sat in a cafe outside and loved the vibe. But very small town.
Williamstown is close to Vermont too.
If you love nature and like the small campus, I would choose Williams, but that’s me. I also love Philly!