I attended a course today in which I ran a training session on Google Sites. My presentation followed an earlier one on Wikispaces, with both presentations aimed at introducing teachers to a means of having an online presence.
Having experimented with both I have a strong preference for Google Sites. I find the whole interface more user friendly and intuitive. My preference is reflected in the fact that during the Wikipedia presentation I found that I had already set up a long forgotten Wikispace for my school, before moving onto Sites, which is the platform that I use for our school website and student portfolios.
Another factor that influences my preference is the whole Google package, with Sites tied up with Google Docs, Gmail, Calendar, and so much more.
However, I do acknowledge that there are schools that are doing fantastic things with Wikispaces, one in particular being Apiti School. Whatever your preference is, it is important that something is used, as an online presence is a great way to share what is going on in your school.
For anyone interested in using sites, I have set up a number of lesson plans to help you get started. The lessons are specific to my school's Google Apps accounts, but from about step 6 for each lesson plan you will find that they are applicable to any Google Account.