Aqua is great—her and Larxene are my fav female characters in the franchise by far.
but I will not allow Terra to be simply written off as repeat Riku. Terra is miles dumber than Riku and I love him dearly for it.
Terra is so much dumber than Riku. I actually don't find the Terra/Riku relation to be that convincing on more than a surface level.
Personally, the BbS trio read like a distortion of the main Kingdom trio.
Aqua is much more akin to Riku, especially if you take his KH2 arc into account. She is constantly doing what she thinks is best for others even when it's clearly taking an emotional toll on her. She's drenched in light, but the Fairy Godmother warns her that a light that bright comes at the cost of an equally dark shadow. She promptly ignores this and stumbles her way through doing the right thing at the cost of removing herself, ultimately paying the price of her removal from the story entirely until KH3.
Terra is the Sora. Dumb, but earnest. Wants to do whatever is needed to protect everyone but has to rely on others for his strength. Except, instead of Mickey, Donald, and Goofy he gets Xehanort, Malificent, and Braig. He does control darkness, and it costs Eraqus's life, but did it to protect Ven at the cost of his own body, just as Sora did for Kairi. And yet, like Sora, a part of him still remains after losing it and continues to grow because of how powerful and raw those emotions are.
Then there's Ventus as Kairi, the amnesiac kid who is new to the group but quickly warms his way into the hearts of the two closest to him. He wants to prove himself and seems to be a being of pure light, albeit one created artificially. He is in way over his head and his introduction causes the two others to ultimately leave their natural home. He tries to fight, and does some things, but ultimately ends up in a coma while someone who looks like him gets substantially more character development and resonance to the story, despite being constantly hinted at as important to the lore.
To answer the OP directly:
I really like Aqua, but like the rest of the BbS crew, she feels like she's a character in an allegory rather than someone making independent decisions whose actions create the plot around them. Their dialogue all feels stunted outside of Ven, but I think that's solely due to McCartney being used to the series and not having to change too much, although his inflections are distinct. He's genuinely great.
I don't have much experience with Aqua's VA, but I know Terra's distractingly as Logan Echolls from Veronica Mars and I know that's not what his actual range is. You even see it with characters from the series. I used to think Kairi had a lot of spunk, but that was 100% due to Hayden Panetierre somehow taking a pile of nothing and weaving it into existence. And that's not a knock on Stoner, who does phenomenal work as Xion, probably my favorite KH character, but there is depth contained within Xion compared to the puddle christened "Kairi".
All this is to say that when I think of Aqua, I think of her failing into the darkness as she awkwardly tells Ven not to worry. It's not her fault in the least, but the delivery with what's happening on screen was that jarring when playing it. She's super fun to play as, though. Easily the best of the characters, even if I spammed mines on my critical run.