Building a feed service, I am on the fence with which I should use. The purest in me want to push and promote ATOM, but RSS is widely adopted. Maybe the simple answer is I offer both. If I go with both do I push my users along and default to ATOM, or have that if users ask for it.
Throughts?
Support both as it will ultimately be up to the consumer of your data what is needed/required/preferred. No reason not to publish both!
Atom. It is widely adopted, and much more clearly specified than the half dozen or so flavours of RSS.
User agents which support RSS but not Atom are rare these days. Giving users the choice between two arcane buzzwords (from their perspective) is just going to confuse them.
Actually, you could use atom and label it as RSS, considering that almost every reader supports atom and because RSS is the buzzword.
You could even have a "alternate link", for people that can't use atom.
I went through the same thought process 12 months ago.
Go for Atom, the spec is mature and bomb-proof (http://www.atomenabled.org/developers/syndication/atom-format-spec.php), and will lead you through the production of tidy, easily manipulated feeds (rather than coming up with your own way to implement your requirement using RSS).
Since Atom is just one spec, it's also easier to implement library support for it - so there are a range of open source Atom management libraries out there, from Apache Abdera (Java) through to Argotic (.Net).
This site has a great, simple description of one major thing that Atom does really well that RSS can't do: http://blog.unto.net/work/on-rss-and-atom/
The author concludes, "I’ve been consistently impressed with how well the authors of the Atom syndication format anticipated the needs of the advanced content syndication community."
His article also links to a detailed comparison of RSS 2.0 and Atom 1.0 that's well worth reading.
In short, even if RSS does everything you need to do right now, it may not do everything you'll want to do next year... so plan ahead and go with Atom.