Costs will surely be the primary driver in determining which aircraft are chosen as has been correctly stated in previous posts. However, both manufacturers know this and will be offering the lowest prices commensurate with sound business practice.
One imagines that for small, ad hoc operators , used 737-300s and 737-400s would be attractive.The scenario is different for an airline with such growth potential as easyjet.
Notwithstanding Boeings commendable re-hash of an aging design,the A320 family offers more space,(width and headroom particularly), and thereby greater passenger appeal. It is also a bit quieter in the cabin and cruises a little faster.
Most importantly though there are the advantages of flexibility of aircraft size, in particular the A321 which offers more seating than any 737 variant,whilst providing outstanding fuel economy.
One imagines that easyjet might already benefit from the extra seats for flights to popular destinations such as Malaga during busy periods.
With auxillary centre tanks fitted the 321 will perform a 6 hour sector with a full load of passengers leaving typical reserves.Therefore it is capable of flying from Scotland to Tenerife even when hampered by considerable headwinds.Capacious,long-legged, yet economical aircraft like this have been proven popular in the UK market.The 757 is a good example.
Lufthansa and BMI have borne the disadvantages of operating both fleets of 737s and 320 family simultaneously.You already know what those two operators have decided for the near and long - term future, dont be too surprised if easyjet do the same!