If we're going by battle Royale (cause it's the only reason Fortnite is even relevant) then it's completely open for debate. PUBG has the bigger fanbase even though it's restricted on less platforms. Furthermore it actually costs money where as Fortnite doesn't. As for which would kill the other: I don't think it'll happen nor is it necessary. I'd much rather both do their own thing as they offer different things. As others stated, PUBG is the more realistic game with more weapon options and such. Fortnite is the more arcade style game with building being one of its main draws. Fortnite is also vastly superior technicality wise, PUBG is a mess on that and it doesn't optimize well. It's the main reason I don't play it as much as I do Fortnite.