You can also do this with ant contrib's if task.
<if>
<equals arg1="${condition}" arg2="true"/>
<then>
<copy file="${some.dir}/file" todir="${another.dir}"/>
</then>
<elseif>
<equals arg1="${condition}" arg2="false"/>
<then>
<copy file="${some.dir}/differentFile" todir="${another.dir}"/>
</then>
</elseif>
<else>
<echo message="Condition was neither true nor false"/>
</else>
</if>