What kind of service does AA have out of Newark? I read something this week, in a story about UA slots there, that AA was the second largest carrier with something over 70 departures/arrivals per day. Do they just not fly nonstop to the destinations you want to go to from there?
Personally, I'd never make the trip to LGA or JFK from New Jersey - just too much potential for obstacles to be thrown in your way on the way to the airport. Think of the travel there during heavy rain or even snow. Throw in an overturned big rig or stalled bus. Why would you want to put yourself through that? I've done that trip a few times visiting relatives in NJ and then out to Long Island, and have to say I've never enjoyed it, but maybe I'm spoiled by wider freeways out west.
One thing you don't want to lose is those UA lifetime miles. Even if you make the move to AA, find something to credit to UA every 18 months (UA mall purchase, charitable contribution to one of their preferred charities, or even a flight).
Tom in Arlington VA tonight