I live about halfway between EWR and PHL. I MUCH prefer to fly out of PHL. At EWR, the staff are generally miserable, nasty people. The airport is crowded, dirty, and miserably in need of an update. At PHL, you generally find nice people. The airport is much larger (at least it feels that way) and no where near as crowded. Additionally, even though it is an International airport, unlike EWR you don't get the typical stuff, like 18 family membres crowded around the security checkpoint waiting for someone who is still sitting on a plane overseas, making passing through to get to your flight or bags almost impossible.
PHL is much cleaner, and the airport amenities are definitely better.
Parking is cheaper as well, you can find it for as low as about $5 per day!
I can't tell you if two hours is enough time, I haven't flown out of PHL since before 9-11. But I can tell you that in my experience PHL is a far superior airport over EWR. Which has got to be the worst, except for LGA.
I've also had pretty good overall experiences with USAir. No carrier is perfect. And I've had problems with US here and there. But they've always been responsive and sincerely apologetic, and done whatever they could to make it right, without an attitude.
Anne