I like both but since most have been pimping Penn in this thread, I’ll say something for Williams:
Williams offers tutorials, for which they are famous. You’re not going to get that kind of personal interaction with profs at Penn, as great as Penn is academically (and it’s outstanding).
Dropping the pom pons–
If you cannot decide based on fit, then decide based on cost. And don’t forget to include travel costs.
Williams is the top LAC, probably – at least top-four in about everyone impartial’s book – and Penn is a top-10 U. Regardless of your choice, you have a bright future if you work hard.