Well, first off they're actually aimed at something similar but not identical.
Both scrape content from 3-party sites but WPRobot can do it from a way larger number of sources and works directly as a WP plugin, server-side. Conversely AS is a desktop app and supports Blogger and blogs on WP.com (ie: yourblog.wordpress.com) making it the perfect tool to automate FREE autoblogs. You'll actually need both if you want to create autoblogs both on purchased domains (WPRobot) AND free platforms (AS). HTH
P.S. If you enable XML-RPC AS will be able to post on standard WP blogs (those on your domains, not the free ones) too...