Google News and Yahoo News are the two largest news aggregators (and news search engines) in the world. Both Google and Yahoo earn their money mainly through advertising, and these websites have so much traffic and so many page views that any downtime they have will be very expensive since it becomes the equivalent of lost advertising revenue.
First things first, the headline of this post is of course not referring to the reliability of the news items or the searches, but of the websites themselves. When looking at the availability of these websites from January through September of 2007, Yahoo News has had more than twice the downtime of Google News.
In this case, the amount of downtime (1 hour and 27 minutes for Google News and 3 hours and 6 minutes for Yahoo News) is not so large that it would be very bothersome to individual website visitors, but it should be all the more bothersome to Google and Yahoo because of lost advertising exposure.