Both are very good modes but it differ from player to player. The one which are below the diamond league, they are not gonna like the FFA because they are not that much upgraded yet. But for the players above diamond will love it as it can be a platform to prove themselves.
Talking about king of the hill it is good for the lower league players. this mode will not get that much liked by the champs and master leagued players.
In my opinion i like FFA more than King of hill because i am An expert leagued player and i will like to prove myself solo.
So better to implement FFA first because it will be more loved than King of the hill