looking for schemas info for a new application

["Robert P. J. Day" <rpjday@mindspring.com>]

  this is undoubtedly off-topic, but i suspect numerous 
folks here can point me in the right direction.

  a friend is designing an XML app -- company invoice --
and i'm interested in what people recommend as the choice
for DTD or schema.

  obviously, validating against a DTD would be simple and
straightforward, but it would be nice to use a schema of
some sort for more comprehensive validation.

  of all the choices, which have decent support, which
look like they'll be around for a while?  XML schema?
TREX?  RELAX NG?  and so on.

  pointers to the appropriate mailing list/web site
appreciated.

rday