Southwest Airlines vs Spirit Airlines
Southwest and Spirit are both US low-cost carriers, but they are not the same kind of airline. Southwest is much larger, runs an all-737 fleet, and has a broad domestic network; Spirit is smaller, Airbus-based, and built around a more unbundled ultra-low-cost model.
Fleet comparison
Southwest is dramatically larger in FlightQ's corrected fleet data and operates only Boeing 737-family aircraft. Spirit is Airbus-based, with A320-family aircraft across a smaller network.
Fare model
Southwest and Spirit can both be affordable, but they price trips differently. Spirit's base fare can be very low but more unbundled; Southwest's product is usually simpler for travelers who value included basics and a large domestic schedule.
Route fit
Southwest is usually the better default when schedule depth and domestic network breadth matter. Spirit is worth comparing when it has a nonstop, the total fare after fees is still meaningfully lower, and the schedule is acceptable.
Choose Southwest for network breadth, simple domestic utility, and an all-737 operation. Choose Spirit when the fully priced itinerary is cheaper enough to justify the more unbundled experience.