Palkia

Dragon tail has better damage and energy gain per second.

Assuming the comparison is similar to Dragonite: Dragon Breath is better if you are trying to dodge all attacks, but Dragon Tail is better if you are just trying to dodge charge moves or not dodging.

I couldn't see an occasion when Dragon Breath would be advantageous so TM'd mine to tail.

Dragon Tail is much better for Raids and Gyms.