Neither of them are really built for speed and both favor physical attacks more than special attacks. If you're going for offensive, I'd say Adamant would be best for both. You could save Jolly ones as well, though I don't really recommend it. You could try Impish and/or Careful, but I don't think either of them are really ideal for defensive 'mons either.
Adamant seems best for Incineroar, but Decidueye was built to be a mixed attacker. Depending on what you want to boost and sacrifice, I would recomment the following natures for Owl Friend:
Lonely: More attack for less defense (its defense is lower than its special defense, so take that for what you will) Naughty: More attack for less special defense Mild: More special attack for less defense Rash: More special attack for less special defense Hasty: More speed for less defense Naive: More speed for less special defense
Spirit Shackle is a physical move and its special movepool is actually pretty shallow, so we might want to go with an attack-boosting nature. Also keep in mind Decidueye can learn both Swords Dance and Nasty Plot.