Dragon Rage is always better for Gyarados, though isn't the best for it later on. Ditch Twister. Dragon Rage always deals 40 damage, no matter the Pokémon. Twister, meanwhile, is a 40 BP move that works off Gyarados' Special Attack and is only super-effective against a handful of Pokémon in the game.
Keep Dragon Rage early on, as it can reliably 2HKO most Pokémon at the level you get it. It becomes outclassed quickly however, so replace it with a more reliable TM move like Return, Dig, Secret Power, etc.