What is the correct MIME type to use for an RSS feed?

Neither. It's application/rss+xml http://www.rssboard.org/rss-mime-type-application.txt

Other commenters have pointed out that the single correct mime type is application/rss+xml,.

However, if you are setting an accept header for a client then

Accept: application/rss+xml, application/rdf+xml;q=0.8, application/atom+xml;q=0.6, application/xml;q=0.4, text/xml;q=0.4

might be a good choice, as it states that it accepts RSS, Atom, and XML (in descending order or preference).