Shadow Ball is better for Focus Blast Mewtwo (A niche that other mons can't really fill well), Focus Blast Alakazam, Focus Blast Gengar, and for neutral coverage in gym defense.
Dazzling Gleam is better for Dragonite/Rayquaza, Blissey, Machamp, neutral offense, and meta-coverage in gym defense.
I would go for Dazzling Gleam, especially if you have a Mewtwo. Shadow Ball is an option with different uses, though Dazzling Gleam can also perform Shadow Ball's uses, just to a much lower extent.
