Qual é a diferença prática entre esses dois:

<xs:element name="A">
 <xs:complexType mixed="true">
  <xs:attribute name="att" type="xs:boolean"/>
 </xs:complexType>
</xs:element>

<xs:element name="B">
 <xs:complexType>
  <xs:simpleContent>
   <xs:extension base="xs:string">
    <xs:attribute name="att" type="xs:boolean"/>
   </xs:extension>
  </xs:simpleContent>
 </xs:complexType>
</xs:element>