Actually, both exams are at same level (i.e. pre-requisites are same). Only difference is - one exam is for JEE 5 and other is for JEE 6.
You can opt any exam, as you are eligible for both. However, going for JEE 6 certification is better choice for below reasons:
1) It is based on latest JEE version (version 6)
2) It is totally focused on EJB. Please note that for JEE6, Oracle has provided difference certifications - 1 for EJB and 1 for JPA. So, OCPEJBD is purely EJB (no JPA stuff).
I think it is called OCEEJD . The first 3 letters are either OCA, OCP, or OCE . Why would you need
professional EJB when you are already professional Java programmer?
And the 3 letters follows are Enterprise JavaBeans Developer .
So OCEEJD .