Both are a form of channel surfing the web for content
Google is a more of a librarian, while StumbleUpon is more TV/internet idea (however everything you watch is already preferred by either you or others like you SU’s is smaller and building itself up, but could be the future of viewing video from many sites (i.e. Stumble Upon video) Both allow advertising in the index, and how users interact with/rate these paid listings affect how often these ads show