The University of Pennsylvania began keeping computerized records about the time of the introduction of Library of Congress cataloguing in 1968, so the vast majority of what has been acquired in that time is in the on-line system. Much earlier material has also been added, but coverage is very far from complete. The card catalogue was "closed" in 1985, which means that no changes have been made since.
This creates a dicey situation. About 20% of the material in the library's possession is only catalogued on the paper cards; but much of what is on the paper cards is perforce no longer accurate (items lost or recatalogued since the catealogue was closed, records of holdings of volumes of sets and journals, etc.). So you should use the e-catalogue by preference, but remember to use the p-catalogue for older materials; and then develop a canny sense of when and how to move back and forth between the two. There are plans, of course, to complete the e-ification of the catalogue, but when it's part of an ambitious and important program called "Access 2000" you know not to expect it next week.