I jusft flew JetBlue BOS to MCO, and returned on Song. Here are my observations. Fortunately, my Song plane was one that had the new IFE. A Song flight attendant said that about 6 of the 36 Song planes now have the IFE installed.

JetBlue: All planes have the DirectTv. There is more leg room behind the exit row. Not sure how the seat pitch compares to Song, especially B6 rear cabin? Staff very friendly and professional. I had to change my flight, and B6 over the phone offered to waive the change fee even though I had not really asked them too. Even though B6 operating out of BOS for less than a week, you would not have known. BOS gate agent got on the PA and made a joike that we were bound for DTW. Also before closing the door the agent thanked all customers for flying B6, and said that they were pleased to be in Boston to serve us. I spoke to the person checking me in, and they came from another airline and indicated the staff was a mix of former other airline agents as well as people new to aviation. Snacks offered were two kinds of chips, biscody, oatmeal chocolate chip cookies, animal crackers, and maybe one or two other items. I had coffee (which was good) as well as OJ. Did not check out the available cocktails.
Key Disadvantages:
*No web check in means you must wait in a ticket counter line;
*Limited route network for flying and redeeming awards; and
*No food available besides snacks.

Song: I checked in at the Delta ticket counter. I was charged the $25 change fee, but the agent was very friendly and helpful. The gate staff did not seem as friendly. They seemed to be main line agents trying to be like JetBlue. THey made some jokes about how cold it was going to be in BOS. This plane did have the IFE which was good. You need to use a touch screen in order to change from music to tv, and it appears you cannot make these changes from the arm rest console. Song supplies ear bud headsets, where as B6 supplies a simple over the head set. Again, the FA's seemed to be main line wanting to be different. One announcement they made was that the plane had this new cool entertainment system, but they did not want the passengers to forget about the crew. Also they tried to make a joke about how Song usually had the best flight attendants in the industry, but those had not boarded. Song offers free CocaCola products, as well as water. However, you must buy any and all food you may want to eat. I purchassed an Asian chicken salad, Pringles chips, and two Song Sunrises (mixed drinks). Also got one small bottle of water for free, and the FA did not charge me for one mixed drink. Total bill I paid was $14. Also with Song you can pre-order meals and browse menus on the web site. FA's also provided a wet napkin with plastic cuttlery.
Key Disadvantages:
*Staff still evolving from main line big six mentality. E.g. hit or miss service especially on the ground;
*For discounted Song fares, you earn only half Delta MQM credit for Medallion requallification;
*No snack of any kind for free; and
*The IFE is more involved to start up, and you must use a touch screen to access many functions.