We have flown both, but prefer Song. Mainly, I really like the option of listening to music instead of TV only (my 7 yr old DS likes listening to the Opera loop the whole way; go figure). And now there is the added bonus of Magical Express; while you could use either on the flight down, you would need to be on an approved airline for the return, and Delta, including Song, is one of the approved airlines. I love the idea of checking my bags at the desk at BWV and not worrying about them until we get back to Boston.
I do like JetBlue quite a bit tho, and will certainly be flying them in the future. I just prefer Song on balance. I think my price point is in the $30-$50 per ticket range for Orlando bcs of the convenience of Magical Express, probably lower for other destinations. But Song was cheaper for our October tix, so that decision was made easier. But I'd fly JetBlue again in a heartbeat, and would not hesitate to recommend it to family and friends.
On major plus in JetBlue's favor is ease of Frequent Flyer redemptions: it is much easier to get a good flight for your airmiles on JetBlue than on the legacy carriers like Delta (who always seem to want to loop me thru at least three cities for that free ticket; JetBlue I can fly direct).