This is a little late, but we recently flew out to San Diego via JFK with Delta, and returned direct from LAX to LHR with VS (both times Economy). Our experience was that on the international leg of the DL flight the seats were more comfortable and we had a little more leg room than on VS, but the food on DL was absolutely awful. However, the CC were very friendly and helpful. On the inbound VS flight we were definitely more cramped, but the food was much, much better. CC were a little surly despite most PAX in our area seeming to be very polite.
The domestic DL flight from JFK to SAN was very poor; seats had no proper support and they don't offer food beyond pretzels even though the flight time was quite long. But as you'd only be going as far as JFK, I'd say DL had the slight edge in terms of comfort and customer service. Just pack your own lunch
