Fortnite is a third-person shooter game with only two modes. One being free-to-play. The mode you have to buy is called "Save the World". And in it, you fight off zombies while building forts. In the free-to-play mode, "Battle Royale", you have to eliminate other players until you're the last man standing.
These two modes can be fun, but they can get boring to a lot of people very quick.
Minecraft on the other hand is a sandbox game with two main modes. One being "survival", where you have to fight monsters and gather resources to survive. And the other being "creative", where you are invincible, able to fly, and have unlimited resources, allowing you to build anything you want.
In Minecraft, you can do whatever the hell you want. The only limits are your own imagination.
From an objective standpoint, Minecraft is a better game.