According to flightstats.com, airport scorecards:
EWR
Departures - 62% on time average
Arrivals - 57% on time average
PHL
Departures - 74% on time average
Arrivals - 67% on time average
I honestly have had pretty good experiences with EWR, but what is most important is how the airline handles customers during IRROPS. I believe CO to be better at this than US.