I've created both a blog and a wiki this past week for my Web 2.0 class, and part of the 1st assignment is to compare the two methods of online communication. In my opinion, a blog is individual and personal. It's used by the blogger to share messages, opinions, recipes, crafts, photographs, and anything else that is of interest to her.
On the other hand, I see a wiki as more of a tool to use by a group (e.g., a class, a team, a company dept., etc.) to share useful information of interest to all members of said group. As such, it is more objective, less emotional, and much less fun than a blog. In a nutshell - blog=fun, wiki=function!