we've flown both out of JFK. I prefer JetBlue for a few reasons:
Leg room is better on JetBlue
The walk to the gate is VERY LONG for Delta flights
If the price of the flight goes down, the change fee on JB is only $80 vs $200 for Delta. We have gotten travelbank credit several times on JB to use for future flights.
At least in the Fall, on weekend flights, JB has a late flight leaving SXM at around 6pm, allowing us extra island time.