I will slightly disagree with your last statements about Penn. University City used to be an unsafe place back in the day, but it's actually a pretty good area of the city as of now (compare it's location to Temple Med for instance). Penn has the largest university police department in the United States, and they actually position officers on most street corners at night. There have not been any stabbings in U.City recently to my knowledge, although there was a biomedical graduate student who was bizarrely mugged and beaten at like 7 AM in the morning a month or so ago, but this is becoming very rare. I walk around U.City and West Philly (and the medical school...which isn't on the outskirts but actually right next to the central area of campus) late at night if I'm in lab late, and I never have felt unsafe. Philly is becoming extremely gentrified and the great feel of Center City is slowly pushing it's ways Northward, Southward, and Westward. About 10 years ago 40th street was the cut off for safe neighborhoods, but this has largely stretched out towards 46th/47th street as students continue to take up housing there. 40th street itself now houses a stretch of hip restaurants (including a Jose Garces restaurant...Iron Chef anyone?), new condominiums, a luxury movie theatre, etc.
Most Penn students lives in Rittenhouse Square/Graduate Hospital...and about 1/3 will live in West Philly/U.City....more so for the M1 year as many will move over from U.City to Center City afterwards. You can walk just about anyplace in Center City from Rittenhouse...catch a trolley, subway, or cab (they're relatively inexpensive if you're in a group of people) to other areas of the city. Philly is relatively car friendly if you live in the right area, so I tend to drive most places that are farther (Old City, Northern Liberties) if I go out there. If drinking is in order, there are night buses at night that send me home to the other end of Center City in about 10 minutes. If you're in a group of people though, again...cab prices are cheap and about equal to the $2/person bus fare.