Wikis have several advantages over blogs. Let's list them all here in a side-by-side comparison. Because we English readers follow text from left to right, I thought the Wiki attributes should appear "first".

WIKI	BLOG
Writers:
Many authors, various personalities & experiences, opinions face scrutiny of community or are listed as such.	Typically one author, one personality, one opinion.
Knowledge comes from community of dozens or even thousands of topic experts.	Knowledge limited by single person.
Content:
Growsrapidly at all hours of the day.	Grows slowly, one post at a time.
Articles constantly change and continuosly updated.	Each post becomes stale over time.
Discussions can take place on pages or in the discussion forum (at least in Wetpaint wikis).	Discussions take place in the comments of a post, typically approved by blogger.
Setup:
Ranges from easy to setup to very difficult, depending on chosen solution	Ranges from easy to setup to very difficult, depending on chosen solution
Most solutions take care of themselves	Many solutions require constant security updates
Spam policed by thecommunity	Spam policed by the blogger
Publishing very easy, no html needed on some platforms (like Wetpaint)	Publishing very easy, some html knowledge useful
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