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Being that this is a battle based on a subjective media, it should not be judged on personal tastes. It will be judged on things that can be viewed much less subjectively.
Technicality: How well the musicians can play and compose music?
Diversity: How much the style of music varies in the genre?
Influence: How much the music influences culture and other genres of music?
Lyrics: Which has the better lyrics. Based on how eloquent and meaningful the lyrics are?
Loyalty: Which side is more loyal to their fans?