I would go with United. (actually, I would stay with Continental, but that is out of the question)
They have the second largest presence at Newark, with non-stops to hubs (Chicago, San Fran, LA, Denver, and Dulles) and to London (I think).
If you are used to flying f/c with Continental for free, this is going to be an eye-opener when you have to buy those 500 mile certs.