“All your teachers at Williams will be professors; not true of Penn.”
Actually, that is not true. All courses are taught by professors at Penn.
“You will be able to do research directly with a professor in a professors lab, for instance, at Williams, but must wait until you are a graduate student to do that at Penn.”
That is not true either. D got a research assistantship with a professor at Penn as a freshman.
“At Penn the preprofessional students will be cutthroat and see you as competition.”
Wrong again. Students at Penn tend to be cooperative and sociable, not cutthroat. Ask the students.