I've been meaning to write this post for a verrryyyy long time, so long that it should be outdated by now (but, unfortunately, it's not). Here's the deal: I keep running across sites that offer the same exact content as an RSS feed and an Atom feed.
What's the point of this? Making readers choose a feed format is like asking them to choose between an HTML and XHTML version of your site - technically there are differences, but end users shouldn't have to care about this. They just want to subscribe to your words of wisdom without having to deal with the plumbing.
Even worse, I often see RSS and Atom feeds which contain the same content yet identify posts differently (i.e.: the GUID for a post in the RSS feed is different from the ID of the same post in the Atom feed). As a result, feed search engines show posts from both feeds as though they were different, which to end users looks like duplication.
So, if you currently offer multiple feed formats, may I suggest that you stop doing this? Just pick a format - any format. If RSS does what you need, stick with it and dump your Atom feed. If you need the extra features that the Atom format offers, dump your RSS feed. Either way you'll be fine, and your readers will be happier.
And if you're just starting out, pick one format and forget about the other one. It doesn't matter to your readers which one you choose, so there's no need to agonize over the decision (and you can always switch to the other format later on).