Hey all.
I am thinking of using either RSS or Atom in my project, but also "enhancing" the feed with some of my own special attributes specifically used by my project.
So I have two questions:
1) Which is most used of RSS and Atom on the web and by the big sites?
2) Which is most suitable to be build from by adding my own tags?
Update:
So RSS is most used, but I should pick Atom since I need to make my own tweaks on a feed? If RSS is more popular, why not pick that? Why didn't Google pick that?
There was a day when I was really interested in syndication and publishing formats. I knew all the quirks of RSS 0.91/1.0/2.0 and Atom 1.0 (and the 0.3 version). Atom was basically born to create something more complete out of the RSS experience which consisted roughly only on the very specifications of Dave Winer's and Netscape's (now only the RSS 2.0 makes practical sense and its specification is here: http://cyber.law.harvard.edu/rss/rss.html). Atom was started by Sam Ruby, then was adopted and developed by a committee of savvy people and it resulted in two things: an XML based syndication format and a publishing protocol. Since 2005 Atom is an IETF standard and in my opinion more complete and better specified than RSS.
As of adoption I think that in raw numbers RSS is still in advantage. A lot of sites decided to stick with the version they already had in place (RSS) and podcasting is usually done on RSS too. There a ton of websites offering both by the way.
As of expanding the format, your second question, Atom has been created with this in mind so you should go down that route. Google GData format is basically an extension of the Atom format: http://code.google.com/apis/gdata/docs/1.0/elements.html