Usually all international flights arrive at Terminal A in PHL, as i think it's the only terminal with immigration facilities.

Check the arrival times as PHL around your own arrival to estimate the crowd in the arrivals hall. I once was in line for about 1 hour (immigration). But it was me to blame, as I went to the restroom after arrival and before the immigration hall.

Maybe GoEnvoy could be interesting for you as well instead a choice seat. But call the US Reservation, as they charge you in US$, which is cheaper again as being charged in €.