Which one offers more reliable sun protection?

The Drunk Elephant sunscreen is SPF 30, but contains 20% zinc oxide. Whereas the Colorscience is SPF 50, but contains only 12% zinc oxide. Which is more reliable? On this site it says the SPF 30 one is. But I don't get why SPF 30 is better than SPF 50. I can't decide between the two, as they both work for my sensitive skin.