I fly that route quiet a bit. Just stick with the miles upgrade. It can be tough on that flight to get upgraded, or not. Depends. I've seen if fly almost empty, and just as full overnight.
Worth the miles either way. You're showing up at JFK at abouts 7 in the am right? Guarantee yourself some rest. Remember, it's still the domestic FC seats on there (though branded BE) The only true BE seats are the 757 with 4 rows. That leaves in the morning from SEA IIRC.