I'm primarily an AA guy, but as I understand it (from my limited but recent UA experience), LAX-NYC is a three-class service on UA, with the worst class being Economy Plus (a few inches better than most airlines' economy class).

So starting there, assuming you have to do so as a non-elite, wouldn't be too painful. Flying any airline as a non-elite sucks, but it isn't the end of the world if everybody's in E+ from the beginning. A couple of roundtrips and you're elite anyway. Then you can upgrade to J at least occasionally, although I suspect LAX-NYC is full of high elites no matter who you fly.

So looking at the worst case scenario, my Y trips, I think UA has the edge on this one.