UA has bassinets that hook into the bulkheads, but they are poor quality compared to JL and NH (made out of some kind of rubbery/plasticky material and padded with an airline blanket, whereas the JL/NH type is upholstered). The Japanese carriers actually block people from selecting the bulkhead seats unless they are traveling with infants, whereas UA gives those seats to anyone and you can often only get the seats in the middle (thus requiring you to trip over whoever nabbed the aisle seat every time you need to take the baby to the lav, which will be often). I took our baby on UA once, and all I can say is, fool me twice, shame on me...

I have flown tpac on both JL and NH with a lap infant and the service/amenity offering is broadly similar between the two. They are both really good with kids, offering toys, special meals, diapers, etc. The main service distinctions I have noticed: (1) JL provides child seats, and (2) NH has a "special services" check-in counter at NRT which you can use when traveling with small children -- they let you sit down while they check passports, issue boarding passes, etc. Note that in coach, JL is now generally a better bet for comfort, as NH is installing narrower seats on some aircraft.

Downside: not quite as many options to get to PHL on Japanese airlines. You could take either with a connection to a domestic partner at ORD, or fly into JFK and take some kind of ground transportation from there (this might actually be less hassle than dealing with a domestic connection). With JL you could also fly into BOS and connect from there on US. Although there are other options through the West Coast I would NOT recommend them in general, as that routing takes several hours longer, and you would have to spend a lot more time in an inferior domestic seat with inferior domestic service...

I have heard rumors in other threads that AA or JL might start a NRT-PHL nonstop soon. It will definitely not be soon enough to help you with your upcoming flight, though.