Maybe a little info on yourself first and your circumstances will help us understand why you want a bird.
I think everyone is a little confused as to why your not sure on what species your getting.
African greys and cockatoos are completely different. And choosing based on intelligence is not the best idea.
How much research have you done?
Do you know the dietary needs of both the African grey and the cockatoo?
I knew exactly what I was getting a year before I got my Eclectus, and I put a lot of work into finding myself a fantastic breeder, putting a deposit down on Scarlet when she was first born, and then waiting 4 months till she was old enough to actually come home with me. Which included weekly visits to my breeders house to spend a few hours with her so she could warm up to me first. Even that didn't work though, I brought home the devils bird. But now she is my best friend, and she loves me to bits. Luckily I had the patience, time, knowledge, and money for her. Or else it may not have turned out as great as it did.
I am also planning on getting an African grey, I have spent over a hundred dollars buying books on them, hours and hours of research and advice, and made sure I know everything about their diet, and ups and downs.
I don't plan on getting one for at least 6 months, although I am looking for a breeder right now, so I can have a visit, spend some time with the greys, and make sure they are the bird for us.
There is absolutely no way I would bring another parrot into my home without making sure I knew exactly what I was getting myself into, and educating myself fully first.
Alot of people think they can just buy a bird and it will be the perfect bird, easy to maintain and stimulate.
But honestly it's not that easy.
I mean look at Alex the African grey, Pepperberg spent so much time with Alex, along with her students. He still plucked, I believe she worked him so hard he was stressed from all the work. Griffin as well. Makes me sad.
All we are saying is maybe take some time to do some research first, it will pay off in the end trust me!