I’m at SWA, and it’s been great. If the choice were to live in base at SWA or commute to UA, I’d say choose SWA. If you plan to stay where you are and are choosing between driving to work for one or the other, I’d choose UA because of seniority progression driven by retirements instead of mainly just growth, plus variety of fleet types.
Of course anything can happen. An economic downturn could hurt, and SWA has a strong balance sheet. Or maybe they’ll get bought by Berkshire Hathaway and they screw things up. Maybe your base you’re planning to drive to goes away. Maybe UA or SWA merge with someone like JB or Spirit, and seniority progression could change drastically.
Even with the what-if’s my recommendation is the first paragraph.