Spirit is probably the healthiest pound for pound in the U.S. market. Delta is probably the healthiest legacy carrier although they are carrying a lot of debt. SWA may be the financially strongest large U.S. carrier.
In regard to Spirit's contract, yes, their A320 pay rates are middle of the road (better than US Air's current contract). Their work rules are superior in most ways to all legacy carriers and most LCC's (save for some parts of SWAs and FedExs contracts). Their retirement could use some enhancement as they have no B fund or profit sharing.