I'll try and answer your questions - but I'd suggest having a read through the other posts here and following other links to get a balanced view
"what i will gain by doing scdjws?" A Certification that looks good on your CV, it shows you know the web services as implemented by standard J2EE 1.4, and also as a side effect that you know the basics of XML parsing and schemas.
"how good is scdjws over scwcd certification?"
I did scwcd cert about 3 years ago and think it was better for me as it provided a solid foundation for web development with java which has been useful.
With scdjws I feel that the future of webservices is fairly open, the standards that the certification are based on have already been superceded and things like REST offer an attractive alternative to SOAP/WSDL etc. I've spent sometime studying for it, so I'm still going to take it, but I'm going to be reading up about REST as soon as I'm done.
"what is the best book for preparing XML stuff?"
Just read other posts, think most people recommend a book by Richard Monson Haefal (if I've remembered the name correctly) - I sort of wish I'd bought it a while back, but have tried to make do with online resources mainly, and it seems a bit late now to but it. One problem in studying for this cert. is the lack of specific study guides like you get for scwcd - but hopefully it makes it more of an elite club that you join by passing.
"will it be worthfull if i write XML certification prior to scdjws?"
Being confident with XML XML schemas and SAX/DOM parsing is a good start before thinking about scdjws. I was looking at the XML certification, but decided I didn't want to get labeled as an XML expert, and web service certifcation would be more help in getting me the kind of work I'd like to do. For me doing both would have been overkill - one certifcation every 3 years is more than enough