All versions of Blogger should now see a "Site Feed" tab in their Settings window which will allow users to not only enable site feeds, but to choose between RSS and Atom.
Some serious thought has to be given as to whether you want to use standard RSS or Atom to create your site feed. At this point in time, we recommend you select "RSS" as your site feed. The reason? Every aggregator can use standard RSS while not every aggregator can decipher Atom.
Unfortunately, you cannot choose both. That would be the ideal solution: allow your users to decide between which method they want to syndicate your site. On Blogger Blog, we will be doing both. However, that entails publishing each blog twice after resetting RSS versus Atom with each blog publish cycle. A real pain, but we are doing this to be able to test both feeds with different aggregators and other syndication services.
Learn more about Blogger's implementation of Atom HERE. You can see what software supports Atom at AtomEnabled.
NOTE: If you have standard Blogger, it appears that you will not have a choice between RSS and Atom. If you elect to enable site feeds, it will be Atom.