In general terms I think many travelers realize that Allegiant, Frontier, and Spirit are in one group, with American, Delta, and United in another, and Southwest doing their own thing. Like you I'm doubtful they can articulate what is different between airlines within each group or how they compare with B6 or AS.
AA, AS, & DL have a better domestic W/Y+ experience than UA and it's not close IMO. I'd put DL/AS above AA above UA and avoid G4, F9, NK. I'd have no problem trying B6 or MX if they worked with my routing and schedule. International I tend to choose non-NA carriers for better soft products.