Well, lets look at the typings of the five trainers you have to face:
Ice
Water
Fire
Steel
Dragon
Both Pokemon have great Atk and SpAtk stats and here are movesets you can cover both with:
Yveltal @ BlackGlasses
Ability: Dark Aura
EVs: 252 Atk, 252 SpAtk, 4 HP
Nature: Serious
Dragon Claw
Focus Blast
Oblivion Wing
Dark Pulse
Xerneas @ Lum Berry
Ability: Fairy Aura
EVs: 252 Atk, 252 SpAtk, 4 HP
Nature: Serious
Outrage
Close Combat
Horn Leech
Moonblast
These sets are basically the same, a Dragon Move for Drasna, a Fighting move for Wikstrom and Wulfric, a recovery move in case your health gets low, and a STAB Move powered up by their aura.
These sets are the same because their Base Stats are exactly the same. There really is no better Pokemon, but if I had to pick one, it would be Xerneas. It has an edge over most of the Elite Four and Diantha.