Yes, Latios is a Dragon and Psychic type, so Psychic gets STAB. It doesn't matter if Psychic resists itself because the most common Psychic types in Gen 7 OU are Tapu Lele and Mega Latias. You either switch out against Lele since youre hopelessly walled by it, or you use Draco Meteor against Mega Latias if you win the Speed tie or switch if you want to play the Careful game. On an offensive Pokemon, you need all the power you can get, but you need to use that power reliably. Both those moves have 100% accuracy, but Psychic is stronger and doesn't have to rely on hax to be good.