Boeing and Ryanair reached terms on the U.S. manufacturer’s largest ever firm aircraft order from a European airline yesterday. The deal, worth $15.6 billion at current list prices, calls for delivery over five years of 175 new Boeing 737-800s starting in September 2014. The order stands to raise Ryanair’s fleet count to more than 400 by the summer of 2018 from about 300 today.

Appearing at Boeing’s chalet (A321/4) yesterday with Boeing Commercial Airplanes CEO Ray Conner, Ryanair boss Michael O’Leary said he had given a team of negotiators until the end of September to report back to him on a fleet study into the 737 Max 8, as many as 200 of which he said he might order by the end of the year. However, he also said he is giving “serious consideration” to the Airbus A320neo.

After Conner asserted that the Max offers an 8-percent per-seat fuel burn advantage, lower maintenance costs and better dispatch reliability compared with the Neo, the often controversial and entertaining O’Leary preferred to reserve judgment. “I don’t necessarily agree with that,” said O’Leary. “I think it’s inevitable that every manufacturer says they are better. I think in terms of fuel burn the Max and the Neo will be pretty similar. I think what is compelling for an operator or an owner is the Max has more seats.” Designed to carry as many as 189 passengers, the Max promises a nine-seat capacity advantage over the Neo.