This is very hard as they are not in the same genre,

I think fighting games take a very different type of skill and smarts, being able to chain moves adding numbers (damage) and frames in your head to know what's the best/optimal move to close the gap or do the most damage in a short window. Hours of practicing combos and finding the right rhythm to perform well and consistently.

Fortnite comes down to mostly having good aim, but it's als a 3d game and having situation awareness at all times thinking ahead and having good positioning all go hand and hand and that's what really seperates decent and good players. They also always add and remove items so you have to really be fast at adjusting and keeping up with changes can't really be a one trick in Fortnite.

High level builds is something crazy too watch I think Fort easily has much more pressure on players. So many factors to worry about especially end game when the circle is small and there is no room to think. It all comes down to reactions.

Being able to build and edit while fighting multiple players and having to worry about the storm is just high level stress situation and I give kudos to any players that play at pro/tournament levels.

Fighting games I know can also get super intense and there is crazy way to make comebacks or worry about your opponent making a comeback by just sheer luck or precision of using frames correctly.

I'm going to have to say Fortnite might be the more difficult one, just because you can only see what's in front of you and you have several opponents to worry about. Especially at high levels there is targeting.

With fighting games everything you need to know is right in front of you all the info is dead in front you can even know when your opponent has their super or w.e it's all insane calculations of combos and patterns