The RELAX NG versus XSD competition is bogus, consequently treating one 
as the winner and one as the loser at this stage is bogus.

They are both grammar languages, and in many cases you can substitute 
one for the other. In fact, it seems that people rarely use the features 
in one that are not also not in the other, at least for public schemas.

See 
http://www.oreillynet.com/xml/blog/2006/04/reconciling_xsd_and_relax_ng.html
for a reference to Expressiveness and Complexity of XML Schemas 
<http://alpha.uhasselt.be/%7Elucg5503/tods2006.pdf> by Martens, Neven, 
Schwentick and Jan Bex which has a lot of material of interest.  (You 
might even interpret that survey as saying that people are *not* buying 
into the exotic parts of XSD. I'd expect the same of RELAX NG.)

People using DTDs, XSD or RELAX NG are all buying into grammars, with 
all that entails. I know I have a much more XPath versus grammar 
viewpoint than many people, but I think grammars, whether DTD, XSD or 
RELAX NG, will play a decreasingly important part in document processing 
(perhaps XQuery/XLinq/XSLT2 will cause a momentary hump due to vendors 
trying to find markets for their shiny new tools) in the medium term.

Cheers
Rick Jelliffe