/Slightly biased towards Koko for obvious reasons/
The neutral answer is this: They both have different roles to play. Depending on the rest of the team and the tile you need either to play should typically decide which of the two is better. That's the obvious, unbiased answer.
That said, personally I recommend Tapu Koko over Zeroara. The reasons for this are:
Electric Surge: This is a pretty big deal in the current meta game. Electric Terrain grabs crucial KOs that can change the tide of the battle. Underestimating the power of well used terrain effects is suicide in the more competitive battles. Even in the replay The Poipole posted, Zeraora got the upper hand because of the terrain boosted Plasma Claws. Honestly this one reason should be enough, but there's more.
Tapu Koko can play a variety of roles, something that I'm not sure Zeraora can pull off just as easily. While I might stand corrected once it's actually released and the more creative folks come up with sets that defy the norm, the truth is that a casual glance over of the move pool puts the Guardian Deity in a better position. With access to plenty of support moves and reliable recovery, as well as good stats, Koko can simply don on a number of sets to fit a number of purposes. Need a lead? Check. Need an all our physical attacker? Check. Need a late game special sweeper? Check. Need a support /pivot? Check. With access to Nature's Madness, Koko can potentially break deadlocks that Zeraora might have a tougher time with.
Zeraora's single typing might be an advantage or a disadvantage. While one need not fear M-Venusaur or Gengar as much, there's some benefits to having a Fairy typing, the most obvious being the number of resistances to Fighting (and U-turn, but that's hardly notable). Besides, the predominant threat is Earthquake/ Earth Power.
Zeraora doesn't learn Ice Punch, meaning, much like Koko, it relies on HP [Ice] to even think about Landorus T. Which with the Jolly nature you suggest, is pretty weak.
Actually, on second thoughts, Landorus T ought to be a point by itself. It is one of the most used Pokemon in the current meta and it absolutely destroys Zeraora. Intimidate, immunity to Plasma Claws, and a resistance to Close Combat? I'm actually ashamed this point isn't higher on the list.
To sum it up, individually Zeraora might have a slight advantage. However, Koko's utility to team mates in the form of Electric Surge and greater movepool, as well as Landorus T's abundant usage (which Koko doesn't like either, but at least threatens it with Specs /LO HP [Ice] ) makes it clear that Koko will stay in power for a while longer. It's simpler for newer users to use, and is of great help to M-Medicham, and anything that have an electric coverage move that grabs KOs that it might otherwise be unable to. Koko is a better team player, and in competitive battling, that matters. A lot.
