Frontier guy here. At frontier we converted all of our 319neo orders to 320neos. I'm sure that the casm is lower on a 320 than a 319, but I believe that having some 319s in the mix allows the company to serve a wider variety of markets. The 319 is cheaper, burns less fuel and requires 1 less flight attendant (obviously not a big saving) but the 319 could be profitable on a thinner route than a 320. I'm sure that when Frontier switched 319 orders to 320s they reserved the right to go back. Perhaps if the economy turns, they'll exercise that right.
As far as the A220 vs 319Neo, I have to assume that the cost to add a new fleet type would be way more expensive than the incremental increased casm of a 319neo vs the A220. I think spirit was smart to order as they did. It's expensive to run multiple fleet types.