Yesterday came the press release confirming Delta's order for 100 B737-900ER.
In the press release Delta stresses a fuel burn advantage of 15-20% against the B757, B767 and the A320. If fuel is responsible for about 40% of your cash operating costs, this is big and results in 6-8% lower COC's. But to be competitive, Delta has to look what the competitors will do, especially the low cost carriers, which have lower COC's to begin with.
Virgin America and jetblue have ordered the A320neo