[spr93]

JetBlue has an all Airbus and Embraer 175 fleet. No matter what you book on B6 mainline, you're getting a comfortable airliner.

Virgin America had an all-Airbus fleet...until Alaska bought them and ditched the Airbus leases because 'Merica-Seattle-Boeing or something. (I'm sure they justified it as mechanical/maintenance efficiencies from operating a single type, but they made a bad mistake staying all-in on a failing company's product.)

Delta's famously agnostic - they fly whatever is net cheapest for them, even if it's an old airframe (that they own outright) that sucks fuel (rather than a more fuel-efficient plane that they lease). Boeings got cheap after the MAX problems. On the plus side, Delta is a very well run operation with competent maintenance.

And then there's Southwest. All Boeing, bad maintenance history. A culture that hates change and new technology.