In his monthly column for E-Content Magazine, Bob Doyle explains it: a blog is for "the latest news" and a wiki is for "content that may be old but still good". Read "When to Wiki, When to Blog", June 11, 2006.
Revised Here is a quote about wikis and blogs from Bob Doyle in http://www.econtentmag.com/Articles/Column/I-Column-Like-I-CM/When-to-Wiki,-When-to-Blog-16900.htm
When all is said and done, the blog is for the latest news and the wiki is for content that may be old but still good. So if your goal is a knowledge repository, of business best practices or policies and procedures, or your home’s favorite recipes or family records, the wiki is your tool of choice. If your goal is to get out the latest word on what’s happening in your own life or your business, then the natural choice is a blog.
Note: in 2008, Bob Doyle stopped writing for E-Content Magazine and running various community organizations. He turned to philosophy and cosmology.
His website is The Information Philosopher with the supporting i-Phi blog. He is currently a professor in the Astronomy Department at Harvard University. Revised