Recommendation on V4.5 DTD vs. V5.0 RELAX NG

Hello list,

I have been searching through the archives and through the web for about 
2 hours, but could not find a recommendation, so I just ask here:

I want to start a new DocBook project from scratch, using the usual Unix 
toolchain (Emacs, XSLT stylesheets, xsltproc, Apache's FOP etc.).

Which version of DocBook should I use? The DTD or the schema?

I have successfully worked on a DocBook project some yeas ago, where I 
used V4.3 and the toolchain as mentioned above. Is it worth to upgrade 
my knowledge? Are the tools stable enough to not get in trouble with my 
customer?

Thank you for any advice.

Regards,
Joachim Ziegler