Everything else aside... Party of four players. Doesn't matter who you go with. There are five current origin characters and they have said there will be more. Therefore, with a Max party size of four, you will never have the ability to go on any other companion side quests when you are playing the game with three other players.

If one of you is not Wyll, then even meeting him in the game is pointless because you can't do any of his side quest story lines. Doesn't matter if you ever run into his little devil mistress at any point in the game, you're stuck because you didn't pick him as one of your four player characters.

But with a Party Max size of six, you can at least switch out one of the two extras that you have in the party and make sure he is one of them so that you can do his side quests.

Likewise, if you play a party of 4 customs, you can literally have NO origin characters in the party, thus you are unable to do ANY side quests belonging to them.

So, party of 4 locks players out of content. Party of 6 does not.