It depends upon your requirement. DTD is not an XML document and its limited. Schema is a XML document and it is extensible. DTD doesnt support Namespaces but schema does. DTD doesnt have many datatypes where as schema has 44 built in datatypes. and you can define your own datatypes. The following Tutorialgives a good idea of Schema and the difference between them. Hope this helps.
SCJP 1.4, SCDJWS , SCJA<br />I can do ALL things through CHRIST who strengthens me.