There seems to be a slight operating cost advantage for the smaller 737s versus Airbus. On the other hand the 321 seems to work better than the 737-900. So if you want flexibility and fleet commonality Airbus wins.
Denti's point about lease costs is an interesting one. It rather depends what deal you do with the leasing companies. Any idea who ultimately controls the lease cost of the aircraft that are joining the fleet? for example the ones from Alitalia.