Before others get knickers in a twist: it's the E190. Also I think they only have one A321 so far, and I don't know enough to make a statement about part / training compatibility between the A320s and the A321 line.

Also, JetBlue already has two cabin products - the "Even More" seats, which you can book for an extra price, and give you additional legroom comparable to an exit seat, along with boarding first and a special security line with a smaller wait. On the A320 I flew a few weeks ago, the exit rows as well as the first 5 rows in the cabin were premium seats. So now you add in Mint and you basically have three classes of seats on JetBlue