I'm leaning towards atom but I'm not sure.
RSS2 is the modernized version of the legacy format, atom is a more recent competing format. Both are read by all commonly used feed parsers.
You'll run into a fanboy here and there that will try to convince you that one is better than the other. In reality it makes no difference whatsoever. Use the default. Or toss a coin to pick one.
If your question is more like "which format is most widely used and supported?", then I got a pretty good answer for you:
I analyzed the world's 50 most read blogs according to Technorati, and found that 9 (18%) offered Atom feeds while 48 (96%) offered RSS feeds.
I've also never heard about a feed reader that doesn't support RSS (although I haven't tried them all) -- so I would definitely go with RSS instead of Atom, if I had to choose.
The fact of the matter is that people will want to use both for different reasons. Google, for example, converts all RSS feeds to Atom before using them in Google Reader. WordPress, for example, can only read RSS feeds in its built-in feed readers (try importing a Google-ified feed into WordPress and everything breaks!).
But which style you use is a matter of personal preference in the end, so you should offer both. This way, the Atom fans can use your Atom feed, and the RSS fans can use your RSS feed.
Besides, WordPress is capable of generating both ... so why limit your users?
If you are using feedburner you can use SmartFeed which can 'translate' your feed from Atom to Rss so you will not need to provide both formats yourself.