Atom v. RSS
Both Atom and RSS get the job done when I comes to providing feeds of site content to users, but it's decided, Atom wins hands down. Just take a look at RSS 2.0 and Atom 1.0 Compared. I will admit that it may be easier to generate RSS 2.0 because it has fewer requirements (because you get so many standards to choose from - yea!), however, Atom fits very nicely into the XML suite of standards that I have learned, whereas RSS does not.
RSS 2.0 was a much needed 'standard' to combine all the previous RSS versions as well as prepare the way for future formats, like Atom, but it has many shortfalls. It was known to be less than perfect, but instead of creating more confusion with a new version, it locked the standard and declared that any other predecessors should be given a new name. Atom is that new name and has one large shortfall: it is not as widely supported as RSS. However, such a thing is temporary.
Short of not having a namespace (which just urks me), RSS is not intended to include XHTML and shows. That is why I didn't even go there and just have summaries instead of full-text-posts on the feed. Also, the channel element always threw me off - if I squinted, I could see a purpose, but otherwise I found it useless. As a developer using XSLT, RSS made it very difficult to specify times, albeit, a very valid standard of the day. I needed to use EXSLT's format-date function. However, no XSLT processor supports such a thing. So I end up including a XSLT style sheet that implements the function. This isn't that bad, but it hurts to take a 40% performance decrease. What is even better is that I get to put Atom's date format into the format-date function since this is the standard format used in XSLT. Of course, if you are using PHP, you don't care that much, but at least with Atom you don't have to lookup the argument syntax for date().
Atom in general makes much more sense and is more familiar looking then RSS.