The best (and only) two options in University City are the Sheraton (where I stayed for my interview) and the Hilton (where I've also stayed when looking for housing, etc). The Sheraton has computers in the room and has Sangkee Noodle House downstairs (which I'm obsessed with). The Hilton is under construction (when my parents stayed this fall, they got a room that was about to be renovated...and it showed), doesn't have wireless in the rooms, but might be a little "fancier" overall. The Hilton sits on top of the Penn Barnes&Noble and Pod, which is yummy, and Penne, which is not.
Both are about equally far from campus, so I'd just stay at whichever is cheaper. Both will also allow you to leave your suitcase at the front desk during your interview, which was nice -- but you can also leave your suitcase at the front desk at school if you need to leave straight from campus after your interview.
The prices can vary wildly though -- from around $150/night to $350/night (advertised as the Penn Special Events rate--ie for when parents are in town!)
Both are within easy walking distance to campus, which is really nice, and are close to the train station.
There are also options in center city and options out by the airport -- both would require you to take a cab or drive that morning, however. The airport options might be a lot cheaper though!