Overall, both airlines were imperfect but undoubtedly well-intentioned and each had positive aspects that outweighed the bad.
JetBlue fell down in the airport experience, especially at Terminal 5 where the security checkpoint line backed up into the check-in area when there was an entire overflow apparatus waiting to be used. In the air, however, the experience was nothing short of perfect and JetBlue did a better job than larger airlines, like American and United, which aren't blocking any seats or providing an in-flight service.
Even on a short hop from New York to Chicago, snacks and a beverage was provided with the airline finding a way to maintain safety while continuing to give passengers what they expect on a flight.
Though Spirit's airport experience was lacking, too, I was pleasantly surprised by the airline installing plexiglass partitions at check-in and gate counters, floor placards for social distancing, and hand sanitizer stations next to the boarding door.
Onboard the aircraft, the flight attendants were going above and beyond to ensure distancing even when company policy is to sell every seat. The airline also didn't assign me a middle seat just because I didn't pay extra for a seat assignment, which I can't say for American Airlines.
I wouldn't hesitate to fly on either again and from my experience on both Spirit and JetBlue, I don't believe being flying these low-cost airlines is any less safe than the majors as long as the standard precautions are taken like wearing a face covering and maintaining proper hygiene.