1.fortnite is a free game that offers optional in game that can be purchased with money while battleground costs somewhere around $30 and is a one time purchase
2.pubg allows players to drive around its map with vehicles plus allows other players to shoot from the passenger seats (which can allow for intense fights between cars).
-while fortnite doesn't offer any vehicles at least at the moment.
3.pubg weapons mostly focus on precision and realism, making them have harsh recoil which in turn causes players to have to really focus on aiming
While it is important to aim in fortnite, it is not as necessary as it is in pubg
-also fortnite offers more types of weapons like rocket launchers and traps that makes the fighting more unique.
4.pubg shows a realistic battle royale with graphics that compliment it and create an environment of stress that makes players feel like they are actually in a warzone.
on the other side i'd say fortnite is pretty cartoony
5.fortnite allows players to gather resources and build stuff w them whether that would be stairs, platforms, walls etc.