The best yet simple comparison I found between both technologies was this one, from which I'll take a comparison table:
|RSS 2.0||Atom 1.0||Comments|
|rss||-||Vestigial in RSS|
|link||link||Atom defines an extensible family of rel values|
|language||-||Atom uses standard xml:lang attribute|
|managingEditor||author or contributor|| |
|pubDate||published (in entry)||Atom has no feed-level equivalent|
|lastBuildDate (in channel)||updated||RSS has no item-level equivalent|
|image||logo||Atom recommends 2:1 aspect ratio|
|-||icon||As in favicon.ico|
|description||summary and/or content||depending on whether full content is provided|
|enclosure||-||rel="enclosure" on <link> in Atom|
|source||-||rel="via" on <link> in Atom|
|-||source||Container for feed-level metadata to support aggregation|
Please let me give some emphasis on RSS's lastBuildDate against ATOM's updated.
While RSS has a feed-level "last updated date", ATOM has an item-level "last updated date". What does this means? It simply means that while in RSS you can't update an item, in ATOM you can. And notice: when I'm saying you can't I really mean it: it is cahotic to try to predict how you'll circunvent this (or your feed generator) and each feed reader has to do it, so does it on it's own way (which is bad - protocols are made so such things don't happen). Let's think, for instance, in the sweet simple case of a blog and it's posts. Whenever you post something on your blog, a new feed item is created, with a date. Now, one minute after you update it. In Atom, you'll get the same item, updated in it's body and with a new updated time. Easy. Now, on RSS you don't have a way of updating a feed. You can act like it was an Atom feed (I guess that that's what most feed generators are doing nowadays) and just change the pubDate of that item, thus creating another, but simmilar item. Now, if my feed reader knows that the reader will be used to get blog feeds, it can somewhat expect that behaviour, thus only looking to the items on the feed at the moment. That will solve our little update problem, but will limit historical faculties of the reader. Now, if the reader just reads RSS as it is, it sees another item, and so each time you update a post you're creating another item and spamming your readers.
So, in conclusion, specially if you're using feeds to a blog or something simillar in structure, then ATOM should be your choice, not RSS.