well, not so much what one has and one doesn't. They both do blogging very well. But they do them in entirely different ways.

SimpleBlog is a strictly front end - in-context editing approach to blogging. Some clients really resonate with this way of managing content. there is no dashboard permissions to manage and what have you.

ProBlog in contrast, is really intended to be a dashboard managed blogging tool. You do all your editing and managing in the the C5 dashboard environment. This tends to have a more professional blogging vibe to it.

hope this helps,
Chad