Definitely use Braviary over Espeon. Braviary's STAB attacks can cover grass and bug, making it useful in more situations. It's easier to get than Eevee and comes at a higher level than Eevee does. It's also fully evolved when you get it, so you don't need to bother with soothe bells, kelpsy berries, carboses, or anything like that. Braviary learns its STAB attacks, strength and fly, pretty early, while Espeon needs to wait until level 37 to learn psychic. Braviary's ability, defiant, is better than Espeon's synchronize.