This answer has been often asked . You should try to search for similar post in this forum . You will be able to find the search link in the top right of the page .
But in general , it depends upon your motivation to pursue the certification . If you are looking to get a better job , then study the EJB version which used more heavily in local companies . I guess it will be EJB 2.0 .
Else if you want to learn new technology , go for EJB 3.0 .