Using keys in XSLT will be far faster than an XPath expression, especially one with //
which can be very slow to execute and should only be used when necessary.
<xsl:key match="Content" use="@Name" name="MyContentsLookup"/>
...
<xsl:value-of select="key('MyContentsLookup','ClientXML')"/>
An XSLT processor can implement internal search mechanisms to quickly look up a value in tens of thousands of entries, far faster than with XPath.
I've published an overview of XSLT keys here: http://www.CraneSoftwrights.com/resources/xslkeys/index.htm