Actually my question is bad... i am looking for something like: When
  is it good to combine StAX and SAX for reading XML files
Whenever possible I would recommend sticking with the StAX APIs.  They are faster and IMHO are easier to use than SAX.  The exception to this when you are using a ContentHandler to process the XML then SAX is easier.  If you hava a StAX XMLStreamReader and need to interact with a ContentHandler then you could use a Transformer for this.
StAXSource source = new StAXSource(xmlStreamReader);
SAXResult result = new SAXResult(conentHandler);
transformer.transform(source, result);