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

Question

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).

Here's a pragmatic answer: whatever the "correct" answer may be (and clearly there is debate about this), text/xml is the type used by pretty much all the popular feeds out there in the wild.

Here are a few that I checked:

$ for f in \
  https://feeds.feedburner.com/TechCrunch/ \
  http://feeds.bbci.co.uk/news/video_and_audio/news_front_page/rss.xml \
  http://rss.nytimes.com/services/xml/rss/nyt/HomePage.xml \
  https://daringfireball.net/thetalkshow/rss \
  http://www.npr.org/rss/podcast.php?id=381444908 \
  http://feeds.serialpodcast.org/serialpodcast \
  http://podcasts.joerogan.net/feed \
  https://feeds.feedburner.com/thetimferrissshow \
  http://feed.thisamericanlife.org/talpodcast ; do \
  curl -s -I $f | fgrep -i Content-Type: ; done
content-type:text/xml; charset=UTF-8
Content-Type: text/xml
Content-Type: text/xml
Content-Type: text/xml;charset=UTF-8
Content-Type: text/xml;charset=UTF-8
Content-Type: text/xml; charset=UTF-8
Content-Type: text/xml; charset=UTF-8
content-type:text/xml; charset=UTF-8
Content-Type: text/xml; charset=UTF-8

So you can be sure that text/xml will be correctly interpreted by commonly used RSS clients.

The most correct is application/rss+xml

The most compatible is application/xml

According to W3C:

RSS feeds should be served as application/rss+xml (RSS 1.0 is an RDF format, so it may be served as application/rdf+xml instead). Atom feeds should use application/atom+xml. Alternatively, for compatibility with widely-deployed web browsers, any of these feeds can use one of the more general XML types - preferably application/xml.

https://validator.w3.org/feed/docs/warning/UnexpectedContentType.html

Go for MIME application/rss+xml to be safe if you want to make sure your feed is compatible with RSS readers and other scrapers. That's what I use.