I assume that they went with the 318/319 due to faster delivery times. (318/319 is manufactured in Hamburg vice Toulouse for the 320). IIRC, the difference in delivery times was significant when F9 made the change from guppies to fifi. That, and I'm pretty sure that EADS was cutting awesome deals on the 318/319s. I think that F9 got it's financing from GECAS; I seem to recall them switching their order from V2500s to CFM-56s due to incentives from GECAS.
F9ers, feel free to correct me on the above; I'm going off of memory and haven't followed F9 closely enough to know if the above is correct.

I found an old filing indicating deliveries. F9's fiscal year runs Apr-Mar; they're currently in the fourth quarter of Fiscal Year 2008. http://www.frontierairlines.com/secf...6/8k022206.pdf
You can see that the transition to deliveries of the 320 from 318/319's been programmed since at least Feb 2006.