WIKIs
Many authors, various personalities & experiences, opinions face scrutiny of community or are listed as such.
BLOGS
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:


Grows rapidly 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,wikispaces)
Publishing very easy, some html knowledge useful