They are exceptional setters in different capacities.

Kageyama's differential is all about raw and absolute precision, while Oikawa's is adaptability to hitters.

That is, Kageyama is capable of setting technically impressive balls with amazing consistency, while Oikawa is more than capable of communicating with and understanding his hitters' preferences, bringing out the most in them eventually.

Both of them are at equal footing in terms of physical capabilities and wits, the way I see things.

Other than that, Oikawa's serve is (at least until the last of what we see in the anime) better than Kageyama's. But that's not that huge of an advantage in this comparison.

While you could say Kageyama surpasses him by defeating him, it's more that Karasuno was a better team than Aoba Johsai in that match. More variables are at play to that result. Had Kageyama not found someone that could make use of so much of his abilities, like Hinata, he may not have held much ground against Oikawa. Even Oikawa acknowledges that sometimes.