My answer would be: "It depends" There are a few variables at work here:
If you are travelling at a peak time, and are afraid of missing out on good prices/times on JetBlue (thus negating your small credit) I would book Song. I do this for our February School Vacation trips, simply because I know the exact date Song will release flights, and can purchase them as soon as they become available. With JetBlue, it's a guessing game, and I'm not brave enough to wait it out.
If you are travelling at a non-peak time, I would hold out for JetBlue, simply because I enjoy that experience much more (nicer personnel, better TV for this spoiled brat).
All things being equal, I don't think you can go wrong with either option.
And as far as Magical Express, you could still use them coming from Disney, you would just have to keep your luggage with you until you get to the airport and check them there.
Good Luck (I wasn't much help, was I? Sorry)