I have taken the family to Cancun twice, Los Angeles twice, Lihue and St. Louis.
Every time we have flown I have price matched on Delta for my family of six and Southwest was a significant savings. On the first Cancun trip it was almost half the price.
I’m not sure what determines the price for the different markets. I also am sure that the pricing fluctuates.
For us though Southwest has been cheaper. Especially if we time it with a Wanna-get-away sale.