Well, there is a lot to unpack here. lol
I'd like to start off by pointing out that they are NOT the same thing.
Sora's Anti-Form and Rage-Form are a direct manifestation of the fact that was turned into a Heartless, while also working as a gameplay mechanic to counter those abusing the forms to get through the game. While we do have lore-reasons why it exists, it's never addressed in the games by any characters, so how canon it actually is can be questioned as well.
He also never had control over it and cannot access either form out of his own volition, even if he seemed to regain some sense of control in 3 by being able to keep his Keyblade.
Dark or Anti Aqua, whichever name you pick, is just Aqua taken over by Darkness. She NEVER became an actual Heartless (or gave birth to a Nobody, while at that) and we literally beat the Darkness out of her.
That by itself can let us question what exactly happened there; if we go by the way it happened, maybe Aqua was actually Norted in some way or form by Seeker of Darkness when he hit her with that Dark Firaga, but I fully believe that the only reason she was even possessed in the first place is because of the powerful side-effects of spending too much time in the Realm of Darkness.
