After using a wiki with my students last year and Edmodo with my students this year, I have found that I prefer using wikis. While you can blog on both sites, on a wiki you can add pages on the wiki to create portfolios of projects and assignments added throughout the year. Students liked to be able to quickly click on a project page and view a previous project. This was nice to share during conferences. With Edmodo it was hard to find previous projects. You have to scroll through all posts.
Link to my wiki: http://meredithmalone5thgrade.wikispaces.com/