To be honest having flown Delta, Aer Lingus and Continental into New York I'd say let price be your decider.
I wouldn't really rate any of the 3 as being best airlines in the world, and I do think the Ireland to US route seems to get the oldest of all the planes from all 3 companies. To date on all the flights I've taken its been the screens overhead and basic (bad) service.
If I had to choose a worst delta would get my vote!
For convenience I did like flying with continental into Newark as the time from exiting plane, retrieving bags and getting into taxi was a lot shorter than doing the same at JFK.
I'd look at sticking to direct flights (not the same as non-stop, so check that you don't have a shannon) rather than going via London / Paris / Amsterdam, as the Immigration in Dublin is well worth having, a 10 min queue here saves you up to an hour of queueing in a US airport.