Fireball only does 5-10% (probably more than that) more damage then frostbolt last time I checked, so you can get away with spamming frostbolt (the snare is beneficial in pretty much every situation), but ideally you should rotate them. You can max your global burst by using Fire Blast: Fireball-Arcane Barrage-Fire Blast in order should hit the target at the same time, depending on your distance from them.