The same distance from City Hall (seen in the distance). Similar massing and overall proportions. Both were consciously created as frontages for important buildings. Yet one is a bustling and successful public space, and the other has been reduced to a cigarette smoking and bicycle storage pit... with a couple planters hoping, in vain, to reduce its devastating harshness.
I would contend that there are two major contributing factors to these divergent results (and no, the amount of linear space between buildings is not one of them) - 1. the modernist design of Penn Center is about as souless and non-human-scaled as can possibly be, and that eventually takes its toll on a place's attractiveness, once the new-car-smell wears off; 2. South Broad Street is a street, plain and simple. Whereas Penn Center is a midcentury attempt at pedestrianized space (without any ornamentation, passive, or active programming), the constant buzz of traffic (which could probably use some calming) keeps Broad Street interesting and alive. It gets even better when programmed with events like the PIFA Street Fair or the Mummers Parade.