Its obvious the structure is different, you need 2 different parsers! Or 2 packed in one (don't really know how helpful will be that)
Because:
RSS 2.0:
<?xml version="1.0" encoding="utf-8"?>
<rss version="2.0">
<channel>
<title>Example Feed</title>
<description>Insert witty or insightful remark here</description>
<link>http://example.org/</link>
<lastBuildDate>Sat, 13 Dec 2003 18:30:02 GMT</lastBuildDate>
<managingEditor>johndoe@example.com (John Doe)</managingEditor>
<item>
<title>Atom-Powered Robots Run Amok</title>
<link>http://example.org/2003/12/13/atom03</link>
<guid isPermaLink="false">urn:uuid:1225c695-cfb8-4ebb-aaaa-80da344efa6a</guid>
<pubDate>Sat, 13 Dec 2003 18:30:02 GMT</pubDate>
<description>Some text.</description>
</item>
</channel>
</rss>
Atom 1.0:
<?xml version="1.0" encoding="utf-8"?>
<feed xmlns="http://www.w3.org/2005/Atom">
<title>Example Feed</title>
<subtitle>Insert witty or insightful remark here</subtitle>
<link href="http://example.org/"/>
<updated>2003-12-13T18:30:02Z</updated>
<author>
<name>John Doe</name>
<email>johndoe@example.com</email>
</author>
<id>urn:uuid:60a76c80-d399-11d9-b93C-0003939e0af6</id>
<entry>
<title>Atom-Powered Robots Run Amok</title>
<link href="http://example.org/2003/12/13/atom03"/>
<id>urn:uuid:1225c695-cfb8-4ebb-aaaa-80da344efa6a</id>
<updated>2003-12-13T18:30:02Z</updated>
<summary>Some text.</summary>
</entry>
</feed>
Comparison here, the difference problem in your code is that on atom you have entry
not item
as in rss!