<xsl:apply-templates>
 <xsl:if select="condition1">
  <xsl:with-param name="myparam1">
 </xsl:if>
 <xsl:if select="condition2">
  <xsl:with-param name="myparam2">
 </xsl:if>
</xsl:apply-templates>

or should I write a four-cases choose instruction, with four different
options combinations?