They're both great actress. But Kajol is a classic actress and the majority of her movies are smash hits. Despise her long disspearance from the film industry, I know she'll do well because her acting is superb.
If it concerns the storyline of Fanaa and Black, I have to say Fanaa will do better, because the surrounding looks more blooming and colorful and then the sudden twist with Aamir showing his trueself as a terrorist. It just shows that terroist has a heart and feelings and not just those cold hearted one we hear about everyday.
Concerning the acting, I don't think anyone can live up to Kajol. Why do you think she recieve the main role in Kuch Kuch Hota Hai and Khabhi Khushi Kabhie Gham, while in both those movies Rani recieved the supporting role alongside her cousin (Kajol), she's a classic actress just like Mew Lalita (Thai actress).