I just checked Wikipedia and it says Atom's is 'application/atom+xml' (also confirmed in the IANA registry), and RSS's is 'application/rss+xml' (but it's not registered yet, and 'text/xml' is also used widely).
'application/rss+xml' seems to be the best option though in my opinion. The '+xml' in the media type tells (good) parsers to fall back to using an XML parser if they don't understand the 'rss' part, but the 'rss' part provides more accurate information on the content's type for parsers that do understand RSS.
'application/rss+xml' seems to be the best option though in my opinion. The '+xml' in the media type tells (good) parsers to fall back to using an XML parser if they don't understand the 'rss' part, but the 'rss' part provides more accurate information on the content's type for parsers that do understand RSS.
All that said, it's a mess.