When making a decision about an airline, it's wise to check the schedule. Doing that will help you understand what might happen if your flight is canceled. When a flight is canceled, a ultra-low-cost airline like Spirit will offer you two options.
First, they'll refund your money and let you find other transportation. That refund won't be instantaneous and you're going to have to by expensive same-day tickets on a different carrier. The second option will be to put you on the next available flight. Due to their smaller size and large number of destinations served, that next available flight might not be for a day, several days, or in rare occasions a week away.
In contrast, a larger airline like Southwest or Delta are far more likely to have multiple flights every day to a destination. That larger network means there's a greater combination of nonstop and one-stop flights with which to reroute passengers to get them to their destination.
Here are some statistics to illustrate the differences between the ultra-low-cost carriers and their much larger competitors. Ask yourself which airline you'd want to be flying if your flight gets canceled:
Ultra-Low-Cost US Airlines:
- Allegiant: 121 destinations with 84 planes
- Frontier: 104 destinations with 84 planes
- Spirit: 72 destinations with 132 planes
Legacy/Low-Cost US Airlines:
- Alaska: 116 destinations with 330 planes
- American: 350 destinations with 962 planes (updated to fix typo)
- Delta: 325 destinations with 887 planes
- JetBlue: 102 destinations with 253 planes
- Southwest: 99 destinations with 750 planes
- United: 342 destinations with 763 planes
Data taken from Wikipedia on 2/19/19.
Given the numbers, it's much easier for the larger airlines to recover if one of their planes is out of commission or one of their pilots or cabin crew calls in sick.
In a way, this is about risk tolerance (as well as whether you have great travel insurance). Despite the fact that I live in Spirit's headquarters of Fort Lauderdale, I won't fly them because the risk of getting stranded due to a flight cancelation is higher than my risk tolerance. I'd rather pay more to completely avoid the ultra-low-cost airlines. Other people will have a different opinion.