Guardian is a better classic tank than a Champion, because Champions get a lot more offensive and ally buffing abilities, while Guardian has more defensive and enemy debuffing abilities. A Champion can heal their allies after the battle, while a Guardian will just take the hits for them in the first place.
Similarly, Commander vs Bard are more of a different style than one being better. Bard is the best casting buffer because of its spell list, but Commanders are martial buffers, so they can quickly change buffs and react to changes on the battlefield a lot more easily, while still having the actions to engage with the frontline.

With both it's like asking if Sorcerers or Kineticists are better blasters. There isn't really an answer, they do different things with the role.