Certificate compliments to experience.
Demand is for EXPERIENCE. So I won't compare 'SCDJWS vs SCBCD', but would compare 'JWS Vs EJB 3'
JWS Experience - Instant demand
EJB 3 - Big change compare to EJB 2.x - development time is less in EJB 3 but concept wise Session/MDB are same. JPA is new big thing in EJB, so demand will improve in near future.
other points if you are new to JWS & EJB there are books for EJB 3 which are helpful to prepare for cert
there is no single book to cover objectives of JWS cert, so you would need to study it from multiple source
Last but not least : Both are different & mandatory for a J2EE Developer, and you would need both, it's your choice which one you want in your kit first.
Note that EJBs can be used to implement web service endpoints. Already knowing about EJB prior to involving yourself with web services is a benefit, in order to be able to determine why you would want to use an EJB as a web service endpoint.