"I haven't decided yet" is what most people would say. "I haven't decided /it/ yet" is more specific and possibly in reponse to a question asking about a choice that the listener has to make. eg. "What's the cat's name?" "I haven't decided yet." "What's the cat's name?" "I haven't decided it (on the name) yet." They mean the same thing but the 2nd sentence is a more specific answer. However, as interchangeable as they are, the 1st one is more commonly used.

I think most people would usually say "I haven't decided yet". "I haven't decided it yet" sounds awkward and unnatural. Laurence