Now that UA has chosen the 737 MAX, I think now is the time to predict on who do you think will win DL's next order (for the MD-88 replacement).
Regardless of my Boeing preference, I think the Boeing 737 MAX has the overall advantage for DL's next order. As with UA, Boeing cannot afford to let one of their most loyal customers defect to Airbus, as what happened with AA. Boeing won the first round with the 737-900ER, and I am pretty sure that Boeing will offer them another super-sweet deal on 737 MAX aircraft that Airbus cannot match with the A320neo. I know that many Airbus fans love to point out that Richard Anderson was formerly CEO of NW, but everyone should remember that the CEO isn't directly involved in fleet decisions.
As for specific variants, I'd expect the 737 MAX 8 to be the most likely if they are replacing MD-88s. DL may possibly order some 737 MAX 9s as well to replace additional 757s.
NOTE: This is NOT a poll on who do you WANT to win the order, but who do you EXPECT to win the order.