I had not flown on two domestic airlines back-to-back with so much the same, and I found there to be a pretty stark differences.
To San Juan, I took two Delta Boeing 737-900ERs with the newest interior (one was only a few weeks old). I flew from Seattle to Atlanta (shocking), then on to San Juan. On the way home, I took two American 737-800s. One had the Boeing Sky Interior cabin, but still shared entertainment screens. The second was an older 737-800, with no sky interior and also shared screens (but more on that later). I flew out of San Juan, through Miami, and then on to Seattle.
The cost of the tickets were exactly the same: $236 each way. I also earned Alaska Airlines miles for both flights, so I didn’t care about miles on either, nor did I have any status [update: I did not realize that Delta only gave me 50% Alaska miles vs American’s 100%. Still knowing this, it doesn’t change any of my choices or opinions since I am not much of a miles guy]. I was also in window seats and had similar seat-mate setups.
I went into these flights with no expectation of doing a story, but the fact that on similar flights, there was an obvious winner, I became motivated. And yes, you will have to wait until the end to see which airline won — no cheating!