I went with the Phineas ending; I just can't bring myself to play an evil character. But Felix's ending seemed lonely, and it sort of surprised me given the conversations he had with post-vision Max while we were walking around (the background conversations that the companions have, especially when Max is in the party, are really wonderful). At one point, Felix asked Max if he could read some of Max's books from seminary, which Max welcomed, so I thought there'd be an opportunity for Felix to expand his education, and when his ending basically said, "Felix, being too dumb for anything but manual labor, spent the rest of his days working hard alone," my heart cracked for him a little.
Indeed, the various ways that companions interact with each other (and with NPCs) based on where they are in their own arcs is very interesting and layered, and calls for many more play-throughs*. ; ) I have also found it useful to ask my companions when there's a decision to be made; they usually give me the "Middle Path" answer I'm looking for for my character. One thing I've noticed, though, is that on my PS4, I don't get any kind of "companion wants to talk to you" marker. So I periodically have to stop and see if they have something to say.
That's a pity. Because it seems like there's hope for Felix with Nell.
* More walk-throughs -- I'm already playing this game so much. You know you're playing "The Outer Worlds" too much when you sit down to watch "A Christmas Story" with your family and when Ralphie** daydreams about defeating Black Bart & his gang with Old Blue***, you want him to kick into TTD and start popping off heads.
** Even funnier when you realize your Better Half's character in TOW is named "Ralph".
*** I have now named my character's rifle "Old Blue"