Actually, DL's PHL-CDG prices are almost identical to US's in Spring/Summer 2013 - and they use a 757 versus US's 333. Prices will probably stay that way until the seasonal "test the waters" period ends sometime between March and early May. I'd wait until early May before making a reservation. However, both the DL and US flights are frequently nearly full all summer.
Now what you might consider is UA/CO PHL-EWR-CDG and save a whopping $600 !!! Seems well worth tolerating the PHL-EWR commuter flight.