I'm considering flying EWR-PHL-BOS this week, flights 4878 and 558, which is the last EWR-XXX-BOS itinerary of the day. According to usairways.com, 4878 EWR-PHL is cancelled 19% of the time and is delayed 30%+ of the time. (A quick check of flightaware backs this up...miserable performance in the last week.)

If 4878 is badly delayed or cancelled, what kind of reaccomodation should I expect? Can I expect to be put on a UA non-stop that eve? Or am I going to be offered a selection of US/AA flights from EWR/LGA/JFK, which will likely be the next morning?

Does the reason for the delay/cancellation impact the typical reaccomodation?

I have no status with US/AA.