Cleveland won a title last year - once you go beyond their big 3 of Kyrie/Lebron/Love, who else in their starting 5 is as good as either of the Celtics I just listed? Nobody.
I wouldn't necessarily see Horford as a lock for starting for either the Warriors or Cavs, TBH. This has nothing to do with his talent level, per say, but both are situations where he would have to come off the bench.
I suppose he might technically start for the Warriors like Bogut did and Zaza does now, but their core unit only has Green as a big and Dray is a better fit for them.
People for whatever reason like to underestimate how important TT is the Cavs, especially with perimeter-heavy Love as the other big. Without TT clearing the glass and providing second chances on offense, the Cavs don't get their ring.
Maybe Horford starts over Love.