Wikis let the group voice emerge. Many people participate within a given wiki, each with an equal voice in a shared space that anyone can edit. Its a different act of sharing to contribute your words to a page that others can build upon. Our instinct is to at first believe this would create conflict and distrust, but it actually builds trust. Each wiki page reflects the current consensual understanding of a given concept. A page isn't a complete or perfect understanding, information and conditions change too quickly for it to be possible Instead, a little wabi-sabi and trusting others allows something powerful to emerge and stay current -- a simply powerful tool for collaboration.
We aren't the only one to think of the differences between weblogs and wikis as individual and group voices. Elwin Jenkins describes it as weblogs turn individuals into webpages while wikis turn communities into webpages.
There are lots of similarities between the two tools. Both are web native, are easy to use, are link-intensive and encourage sharing.
Both are being widely adopted, wikis less visibly because of private group use and at different paces in different areas. A customer once explained to me how he thought wikis were more popular than weblogs in Asia because group voice is valued greater than individual voice. Regardless of popularity, different cultures and organizations will have different values that is reflected in their tool selection.
Its not a choice between one or another. The temporal structure of weblogs and logical structure of wikis are a complement for lasting effects. One of the more powerful patterns in an organization is how an opportunity is published in blog, possibilities are swarmed upon in blog conversation and then driven to consensus and outcome in a wikified document. After the outcome, the knowledge and its social context remains.
Jim McGee has more here and here. Thanks Dina for the great Wiki links! (she has even more if you visit her blog).