problem with Xpath in Variable filled by choose

I have written following Code:
 
I use XSL v 2.0 and transform it with a JavaScript transformer:
 
<xsl:variable name="CategoryPointer"> 
                 <xsl:choose> 
                      <xsl:when test="$category = 'main'"> 
                           <xsl:copy-of 
select="document('style.xml')/root/DeviceParameters/DesignMood/Category/main
"/>           
                      </xsl:when> 
                      <xsl:when test="$category = 'news'"> 
                           <xsl:value-of
select="document('style.xml')/root/DeviceParameters/DesignMood/Category/news
"/>                
                      </xsl:when> 
                 </xsl:choose> 
            </xsl:variable> 
            <xsl:value-of select="$CategoryPointer/HeadlineColor"/> 
            <xsl:value-of select="$CategoryPointer"/> 
?
   
 
My Problem is here:
 <xsl:value-of select="$CategoryPointer/HeadlineColor"/> 
This row should give me Output of a Node in my XML File there ->
('style.xml')/root/DeviceParameters/DesignMood/Category/main/HeadlineColor 
   
But it seems that here in this following part my Xpath that should be saved
in the variable CategoryPointer  is transformed into a String in this Lines:
<xsl:variable name="CategoryPointer"> 
       <xsl:choose> 
                      <xsl:when test="$category = 'main'"> 
                           <xsl:copy-of 
select="document('style.xml')/root/DeviceParameters/DesignMood/Category/main
"/>           
                      </xsl:when> 
   
So my question: How can I handover a Node to this Variable instead of a
String?
 
Thanks for you Help
 
Dietmar