XML Input -> XML OR Text as output

Hello everyone,

In few words I have this problem:

I have an XML file as input.  Based on a condition I would like to
produce the output in XML or in TEXT.

However the output tag has to be declared in the beginning of the XSLT,
right?

<xsl:output method="xml" version="1.0" encoding="ISO-8859-1"
indent="yes"/>

Or can I set this later within an if-condition?

Many thanks,
Houman