So both are kids and both have a tragic story. Both orphans.
The thing that makes Tina GREAT is that she is a badass, she don’t need anyone saving her, she knows everything about bombs and explosions, thing that makes her USEFUL in BL2 (Mr Torgue is proud), and yeah she is insane but at the end, she loves her friends and have such a great character development in Assault on Dragons Keep.
On the other hand we have Ava that is also an orphan and a brat and that’s it. She is just granted with an immense power because yes and then she is the new leader of the Crimson Raiders because yes.
They could have make the story of a girl, daughter of researchers of sirens that were killed to steal their investigation. So now we have a character with an interesting past that can help us learn more about the sirens because she continues the work of her parents. And at the end of the game, after a GREAT fight, Maya dies and give her the power of the Siren because she deserves it.
Is this something difficult to think? Give the girl a reason to exist other than “yeaah… we need a dramatic moment and some one to give the powers of Maya”
So that is why Tina is great. She deserves the love cuz she just doesn’t exist just for the sake of it. She help us, she is important to the story and have a great personality.