Having flown to WDW (and elsewhere in Florida) a lot out of Logan (we live two miles from Fenway park-there's no reason to ever consider another airport), I advise you in the strongest terms to keep watching Jet Blue fares, and don't be afraid to wait months to actually buy the tickets.
Several reasons:
1) Song is out of business in April. Yes, Delta still runs past then (the Chapter 11 business shouldn't be a worry), but so far Delta's fares out of Logan to Orlando have been nothing to get excited about.
2) I can vouch for the constant Song scheduling changes. We're using up a Song credit this week before it expires, and Song has changed Friday's return flight five times on us since I booked it. Without trying to sound mean, I bought an airline ticket for a certain time because that's the time that works best for me. When you change that time by several hours, it really sets forward a domino effect that screws up a number of other plans I've made. I view my airline ticket like a contract...we agreed that I would pay a certain amount of money, and you agreed that you would fly at a certain time (weather permitting, of course). Don't change the terms of the contract after money has changed hands!
3) IMHO, JetBlue is a much more pleasant experience, especially now that they've moved into terminal C in Logan. They seem to be more consistently on time. All this is personal impressions, of course.
Jet Blue will lower fares as time goes on, but you do have to look at the website daily. Don't expect to ever have to pay more than $79 each way, and you might get it for a tad less by holding out.
Hope this helps!
Off to WDW in 41 hours as I type this!