In-game, either works. Lucario is easier to get, as Korrina gives you one as part of the story, and it can Mega Evolve. Aegislash is also very good however, and both are excellent. I would prefer Lucario, however, for how easily it can be obtained. When Mega Evolved, Lucario has higher Attack than Aegislash, allowing it to dish out more powerful hits. Aegislash has the superior defenses while in Shield Forme, but in-game playthroughs are all about hyper offense.
Lucario @ Lucarionite
Ability: doesn't matter
- Close Combat / Power-Up Punch / Brick Break
- Poison Jab
- Stone Edge / Strength / Swords Dance
Hope I helped!