Both are relatively easy tools which do not require teachers or students to learn any special program tools or computer skills. Their uses and applications are only limited by the vision and purpose for
helping students learn.
I agree that it depends how you're using it, I think for a project, it's best to use wiki where all the students can collaborate together on the same page. I use blogs for student portfolios where I could collaborate with parents and other educators or specialists, which is easier to read as you'll know who saids what. And I could always add 'pages' as a wiki.