Both are ideal for learner-centred activities with learners taking part in collaborative tasks aiding knowledge construction. Both can be used to ‘deliver’ rich content for learners to absorb.
The video below is a fictitious debate between JFK and Richard Nixon on the propriety of using blogs vs. wikis. It may (or may not!) help you choose which to use, or perhaps you will find a combination of the two best suits your educational purposes.
So, what is the difference?
The main difference, then, between a wiki and a blog becomes the layout and organisation of content. With blogs, content is organised according to chronology (most recent articles shown first, usually) and it can be categorised and tagged to make it easier for users to find. This makes blogs good for documenting changes over time and for reflective learning.
The open style of a wiki means that content can be organised in whatever way the users choose – which can make it hard to navigate and make sense of. A lot of learning takes place, however, in wiki tasks in which learners are challenged to organise and present their content in an informative and user-friendly way to an audience.
Another difference is that usually (though it doesn’t have to be like this) a blog has one main (learner) author and other learners visit the blog to leave comments – this approach is good for brainstorming and generating discussion.
With a wiki, however, there is rarely a single author but there are many contributors, all of whom can edit any part of the wiki they choose. The continually modifiable easy-access nature of wikis makes them ideal for group projects and collaborative writing, for example..