I would always recommend you buy the largest and sturdiest cage your space can accomodate. Your bird will be locked in the cage most of it's life, so ample space to be able to flap it's wings without hitting them on something and injuring themselves is important. They NEED to exercise their wings, and they need to use their beaks to explore, climb, and eat so being able to strengthen their beaks and keep them trim is also key.
Imagine if you had to be in one room your whole life, you'd want as much space as possible--just make sure you dont' then try to FILL the space with a zillion toys and "things". Out time is really important as well, so if your bird is tame, you want a cage with a play gym on top!