Flamethrower takes out Ice and Steel types that you're weak to -- but so does Aura Sphere. Psychic allows you to reliably counter Poison types that give Togekiss trouble, but it's going to see less use than Flamethrower/Aura Sphere, as Poison types aren't too common. You should probably use Psychic, as Aura Sphere already takes out all weaknesses Flamethrower can handle, while also providing a way to nail Rock types. Another option would be to swap Aura Sphere with Flamethrower for more power, but you lose your way to hit Rock types super effectively if you do that.
Hope I helped!
