Those that might argue that Soviet Communism was a fascist system would point to the fact that 1) there was an intense cult of personality around the leader(s) and 2) the full powers of the modern state and mass communications were mobilized to promote a state ideology (i.e., what some people call "brainwashing," as well as big brother tactics). I wouldn't make this argument since I think there are aspects of each that make them sufficiently distinct as to not be in the same category, but they certainly aren't polar opposites (as some conventional wisdom might hold).