I agree with almost everything in your post except the part regarding access to research opportunities for undergraduates.
Research opportunities for undergrads should be plentiful at Penn, but paid research opportunities probably are offered to graduate students first. Overall, opportunities for research as an undergraduate should be much more available at Penn than at any LAC.
Of course, any student at almost any college or university can arrange to do independent research. Not a problem at either Penn or Williams. But, depending upon the type of research, the resources should be far superior at Penn than at Williams College or at any LAC.
Because Williams College and the University of Pennsylvania are so different in most respects–including track–you must have a preference–especially if you think that Penn is isolated.