Tapu Koko, it is better because of its ability, unless you are making a VikaBulu deck, it's really up to you.
Thanks! I ended up buying both and putting them both in my deck. I'm a 13 year old Christian, and while my mom doesn't mind and actually let me buy the tins, my dad is still stuck in the 90's "Pokemon is a satanic game for perverts" craze. I have a very limited collection of 2 GX's, around 7 EX's, and around 200 cards besides that. I have a few water energies, quite a few electric energies, tons of grass and psychic energies, and a fair number of ground energies. I have built to expanded format decks, and I don't know what else to do with my collection. My main deck focuses on this strategy:
Get a 3 energy total electric type on my bench
Get Rayquaza out as my active
Get Bulu GX benched
Use Turbo Storm to start building Bulu
Get Koko GX and Brock's Grit in my hand
Use Bulu till it dies, then GX for a full restart
Once Bulu dies, use Koko with Aero Trail to full build
Once Koko dies, use brock's grit to get back Koko, Bulu, and 4 energys.
Hopefully, Rayquaza will still be alive to use turbo storm for a full Bulu build once again.
Is this a good strategy? Sorry for the long post, I'm still new!