Don't know about the overall costs but the field performance of the 737 beats the daylights out of the 320. Exel 737-800s going Skiathos direct to Manchester with a full load of 189 pax and not an A320 in sight.
Seat mile costs and maintenance costs are very similar but it obviously depends on the deal you do with your maintenance supplier. Easyjet can answer that question but then they got a really sweet 320 deal from Airbus after Ryanair turned down the 320 about 5 or 6 years ago. That might tell you something. If there's one thing Michael O'Leary is good at it's reducing costs, so he obviously calculated that the 800 was a better bet! I doubt that he stayed with Boeing for any reason other than it was the cheapest.