Neither have positive connotations, but "fundamentalist" is worse. Fundamentalism specifically invokes ideas of religious fundamentalism, particularly "religious right" style protestant beliefs.

"Radical" on the other hand is used in a more diverse set of cases. It invokes thoughts of any number of political or religious beliefs that are either outside the mainstream or do not conform with progressive western values as they are traditionally stated.

I don't think either is dismissive. Descriptive, with negative connotations, but being critical is not the same as being dismissive.