I would also agree that PHL is the airport to fly into.
If you do take (
SEPTA), realize that you'll need to take the R1 to 30th Street Station (essentially center city Philly) and then catch the R2 south to Newark, DE. Depending on how you tired you are after flight, this could be a brutal stretch of catching multiple commuter trains.
It's a bit of a haul from PHL to Dover, but it may be easier on you than the trains (but maybe not easier on your hosts).