Personally, I think it's worth going through the SCJP exam to get to the SCJWSD. It's an extra test, yes, but it proves you know the basics before you get into something more specialized.
Not that I have anything against IBM's test, but as I've stated in otherplaces, I think the benefit of the SCJWSD exam is that it's fairly agnostic when it comes to tools - it's concentrated on how you do web services with Java, as opposed to IBM's exam that does talk in its objectives about how you're creating a web service with their tool.
Just my $0.02.
Theodore Jonathan Casser
SCJP/SCSNI/SCBCD/SCWCD/SCDJWS/SCMAD/SCEA/MCTS/MCPD... and so many more letters than you can shake a stick at!