Per the McCade study guide, you need to know about both from a high level. EJB 3/JPA for the new code and EJB 2 for legacy systems. The good news is you don't need to know low level details. There's a different exam for that!
Explain the benefits of the EJB 3 development model over previous EJB generations for ease of development including how the EJB container simplifies EJB development.
McCade's Study Guide is good, but you don't have to purchase it to understand what you need to study.
If you read the rest of the objectives under the "Section 4: Business Tier Technologies" heading, you'll realize that you don't need API knowledge , but you need to understand the concepts and applicability of the business tier technologies (EJB2.x, EJB3.x, JPA, Web services etc) clearly.
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