A lot of the posters have some very good points.
Although Yale and Penn are both great schools, they have significantly different vibes on campus. Academically and socially, there are real differences in the schools that can be picked up on during an admit weekend.
Which did your son like better? Because the answer should be pretty clear to him.
And secret societies really don’t play a huge role in campus social life. Almost all are seniors-only, and there’s not a huge divide between people in a society and people not in one. My best friend was in a society, so she’d be busy Sunday night and Thursday night, but it didn’t make a huge difference (and only in senior year), nor did non-society members (vast majority of senior class) feel socially ostracized because they weren’t in one.