The answer changes for everyone, but my personal belief is that the Dystopian future is more interesting. Many people enjoy stories with decent conflict, and a true dystopia has never been experienced by our society (but then again, neither has Utopia). Regardless, people love reading about things they've never experienced. The unknown is usually fascinating, and when conflict is added is promotes interest. Also, dystopias have potential for that "nitty-gritty" feel, or potential for lovable heroes (Think "V for Vendetta).
Utopias are nice to think about, but without conflict, reading about perfect societies can only entertain for so long.