Horner chosing Checo over Sainz is pretty fascinating. I guess I get the rationale - Max is dominant and can carry the team to driver's and constructor's wins as long as Checo isn't too bad. And while he's inconsistent on some tracks he runs second on others while able to co-exist with Max, who gets the designers' full attention. The car is going to be set-up for Max and that's the way it is.
While Sainz may seem to have more up-side than Checo and given how the field is getting more competitive at the front, it might be that Red Bull need more points from their #2 to stay on top on the constructor's. But it doesn't necessarily mean that Carlos would be better in that Red Bull than Checo and Perez is a known commodity both on the track and as a teammate . . . Carlos is the unknown.
Christian chose stability and the known rather than a riskier step into the unknown. But if Ferrari and McLaren continue to contend with both of their cars and Checo continues to have these stretches in the season where he can't qualify and struggles to score points at all, the decision may prove to have been wrong.
Although we may never be able to tell because what's Sainz going to be driving next year? A Sauber/Audi or a Williams? Won't be easy to compare them - but we certainly can compare them now.