You know I think we covered this already but just in case we didn't:

May 22 (Reuters) - Delta Air Lines:
U.S. carrier Delta Air Lines will continue to sit out a wave of orders for the latest fuel-saving narrowbody models produced by Airbus and Boeing as it waits for newly developed engines to prove themselves in service, Chief Executive Richard Anderson said on Wednesday.

The two manufacturers have stacked up orders for more than 3,000 of the revamped models, the Boeing 737 MAX and the Airbus A320neo, which boast fuel savings of around 15 percent from the second half of the decade.

"We'd rather get toward the end of a production line because one, the airplane has probably been stretched, and stretched economics are always better than the original economics," Anderson told Reuters reporters and editors at Reuters New York headquarters.