Thing 17

A wiki is a more focused instrument than a blog is. The wiki typically provides info on a particular topic of interest, whereas a blog might have that topic, but will also have other interests of the blogger.
I think a blog would be more useful for journaling activities, and reflection assignments. A wiki would be more collaborative, much as group projects are now. An assignment on planets could be put together by students as young as 3rd or 4th grade. Social studies assignments could use a wiki. The advantage is that it wouldn't necessarily be limited to in-class time, but could be a homework assignment to which each participant could contribute at their convenience.
The main problem I encountered with the wiki was getting it to look the way I wanted it to. Perhaps this could be solved in a group collaboration by each person contributing their info and then putting it together in a more stylish fashion at the end of the project.