JetBlue flies the A320 and the E190. You can't tell what type of plane you will have based on the flight number as they assign planes based on demand rather than route.

After you enter your personal information (but before purchasing) when buying a ticket you will be presented with a seat selection map where you can pick what seat you want. If the seat map is 4 seats across, you will be on an E190. If it has 6 seats across, you will be on an A320.

If you select a seat in the "extra legroom" section of the plane, you will be asked if you would like to pay the upgrade fee.

In my experience, on an A320, any seat outside of 9A/F and row 10 (which don't recline) is equal. Some planes have a small box under the middle seat to handle TV equipment. For EML seats, I prefer row 3 as it has the best aligned window to your seat (IMHO). If you don't mind having the tray table in your armrest and having to look down to see your TV, row 1 gives you extra legroom and is free (though you need to ask for it at the airport when you check in).

On an E190, EML isn't really worth it. Seats in the back half of the plane have one more inch of legroom than those in the front. Row 11 does not recline.