Tag clouds are lists of keywords linking to specific content such as articles and blog entries. These footers are more indexes of categories. Its more a matter of where the links point. Relevant content good, "administration" and policy type pages not so much.
Thats from the pure
SEO standpoint. From the user standpoint, simplicity is usually best. Tag clouds present a novel way for a user to find content based on a keyword of interest. In the same way, a properly crafted footer presents a lost visitor a way to return to a category index page, without overwhelming that user with a long list of choices.