That seems perfectly reasonalbe to me, you just need to put some logic in the <xsl:for-each> blocks (and then close them). I would recommend using an <xsl:template> to combine the code so you only have to write it once...

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE xsl:stylesheet [ <!ENTITY nbsp "&#x00A0;"> ]>
<xsl:stylesheet 
 version="1.0" 
    xmlns:xsl="http://www.w3.org/1999/XSL/Transform" 
 xmlns:msxml="urn:schemas-microsoft-com:xslt"
  xmlns:umbraco.library="urn:umbraco.library" xmlns:Exslt.ExsltCommon="urn:Exslt.ExsltCommon" xmlns:Exslt.ExsltDatesAndTimes="urn:Exslt.ExsltDatesAndTimes" xmlns:Exslt.ExsltMath="urn:Exslt.ExsltMath" xmlns:Exslt.ExsltRegularExpressions="urn:Exslt.ExsltRegularExpressions" xmlns:Exslt.ExsltStrings="urn:Exslt.ExsltStrings" xmlns:Exslt.ExsltSets="urn:Exslt.ExsltSets" 
  exclude-result-prefixes="msxml umbraco.library Exslt.ExsltCommon Exslt.ExsltDatesAndTimes Exslt.ExsltMath Exslt.ExsltRegularExpressions Exslt.ExsltStrings Exslt.ExsltSets ">

<xsl:output method="xml" omit-xml-declaration="yes"/>
<xsl:param name="currentPage"/>

<xsl:template match="/">
<!-- ... -->
<xsl:choose>
  <xsl:when test="$prev_dates=1">
    <xsl:for-each select="$currentPage/node [string(data [@alias='umbracoNaviHide']) != '1' and(umbraco.library:FormatDateTime(data[@alias='dato'],'yyyyMMdd') &gt;= umbraco.library:FormatDateTime(umbraco.library:CurrentDate(),'yyyyMMdd'))]">
      <xsl:call-template name="doSomething">
        <xsl:with-param name="item" select="."/>
      </xsl:call-template>
    </xsl:for-each>
  </xsl:when>
  <xsl:otherwise>
    <xsl:for-each select="$currentPage/node [string(data [@alias='umbracoNaviHide']) != '1' and(umbraco.library:FormatDateTime(data[@alias='dato'],'yyyyMMdd') &lt; umbraco.library:FormatDateTime(umbraco.library:CurrentDate(),'yyyyMMdd'))]">
      <xsl:call-template name="doSomething">
        <xsl:with-param name="item" select="."/>
      </xsl:call-template>
    </xsl:for-each>
  </xsl:otherwise>
</xsl:choose>
</xsl:template>

<xsl:template name="doSomething"><!-- This just prints the nodeName of item -->
  <xsl:param name="item"/>
  <p><xsl:value-of select="$item/@nodeName"/></p>
</xsl:template>
</xsl:stylesheet>