Well, I'm just back from New York via the AA service. I travelled in Economy, which was full. On both legs of the journey, there were plenty of Americans too - a good sign for route sustainability I'd have thought.
In flight service was pretty good, and it's nice that they don't hard sell you duty free's. They need to sort out the inflight entertainment though - very 1990s. No seat back entertainment, just aisle screens. Doesn't bother me these days as I carry an iPad, but if you don't, the journey could get tedious. Also, no wifi, and there's no plans to retrofit the 757's either. Have never flown on the United service so don't know how this compares.
Coming back, the aisle screens were showing the inflight route to LHR!! That's something they need to fix (although about halfway through they switched it over to say Birmingham).
I was staying in Mid Manhattan, and JFK is a pig to get to - although very fast through immigration compared to EWR. Going out, I got the Air Train, then Long Island Railroad, followed by a subway, and I was in the apartment within 45mins. Heading back yesterday, I got a Cab and it took 1hr 10m!
By contrast, EWR is a straight train in on the New Jersey Transit - although much more expensive in a taxi than JFK due to rules forbidding NYC cabbies from picking up in Newark. For those living on the west side of New York and in New Jersey, this is obviously the much easier airport to use - who wants to cross town to get to JFK? Likewise, who on Long Island or someone heading north of NYC would want to endure the horrendous traffic of NYC to get to EWR?
While I was there, I went down on the train to visit some people at Asbury Park, NJ. To come to the UK, they will either use EWR or PHL - JFK is a bit of a mare for them.
What I'm trying to say in my meandering, jetlagged way, is that I believe that both United and AA services will thrive. They both offer viable services for those wishing to visit NYC, but each offer connectivity to a slightly different catchment area too. They compliment each other nicely.