[eluser]WanWizard[/eluser]
It all depends on the what the object contains, and what you intend to do with it.
In terms of data manipulation and cost (processing, memory), array's are a better choice. Also arrays are simpler to serialize (for example if you store them in a session).
Objects however allow you to embed code as well, protect data structures inside the object, etc. They also allow you to make relations ( $this->that ) which are heavily used by for example ORM implementations.