Gregory's ServerFor me at least blogs have a narrative structure, whereas a wiki can start out as a collection of somewhat fragmentary materials that isn't ready to be told start to finish.
If this was paper I'd say that the wiki for me has felt a lot more like note cards (terse, single topic, incomplete) and the blog is more like little essays that are informed by the note cards but that are narratives that stretch out across a half dozen or more topics in their course.