When developing web services that serve XML, it’s important to get the MIME type right as the wrong one could decoding errors due to default character sets.
text/xml defaults to sending your XML document in the us-ascii character set. See section 3.1 of RFC 3032 for the reason why.
application/xml defaults to sending your XML document in the utf-8 character set. See section 8.10 of RFC 3032 and Appendix F of the XML specification for the reasons why.
Of course, you can override these defaults, but it’s worth remembering if your XML application doesn’t behave quite as you were expecting and is rendering odd characters.