Go where you feel you will have the most opportunity to participate in the schools community at all levels. For me that is far and away Penn. Being larger there is simply less bias ( not the same as no bias) to sheltering and catering to the
lets say more affluent students. Williams , while certainly a great school , fact is just like at top prep schools you will be competing at times against factors well out of your control. OF course if you are among the very affluent disregard the above.