Similarly, will you have checked baggage (which may affect how quickly you get through customs)? If you do, you'll likely have to take time to re-check the bags at the check in counter with your 2nd airline before clearing security.
If I had to choose between those I'd also go SFO. The transfer should be easier, especially if you're cutting the time close - JetBlue operates out of the International Terminal at SFO so you won't have to go that far to reclear security.
At LAX, Spirit operates out of T5 and you'll have to walk/run between terminals (about 10 minutes walking) or take a super slow shuttle bus. (Especially wouldn't recommend this if you have checked bags that you have to carry over to T5)