What is a way to critique the difference between Meggie in The Thorn Birds and Scarlet O'Hara in Gone with the Wind?

In some ways, Meggie and Scarlett would seem to have much in common. Both make bad marriages while secretly being in love with another man. Both are beautiful and both suffer adversity. Both are raised Roman Catholic.
Scarlett, however, is far less passive than Meggie. Scarlett is, in fact, more like Meggie's daughter, Justine, a person who takes hold of life and makes her own experience. What we love about Scarlett is that she is never one to take life lying down or to accept her lot as "fate," as Meggie might. Instead, Scarlett always fights back. Even the ruins left by the loss of the Civil War, the threat of starvation and, later, the possibility of losing Tara can't defeat her. The South might suffer defeat, but not Scarlett. She rises to every occasion and kicks down every barrier in her path. She is one tough woman.
Scarlett marries to save face (Charles), to save Tara (Frank), or to have wealth (Rhett), and uses her marriages to her own advantage. Except for with Rhett, she is the dominant partner in the relationship, and even Rhett initially adores her and caters to her whims. In contrast, Meggie is dominated by and taken advantage of by Luke. She is more likely to be misty, submissive, and romantic about men in contrast to Scarlett's completely hard-headed approach.
It is impossible, for example, to imagine Scarlett putting up with being robbed and exploited by a husband the way Meggie is by Luke. Luke takes Meggie's money and even has her wages diverted to him. We can hardly imagine Scarlett putting up with that situation for more than a few seconds. She'd crush Luke, one way or another.
On the other hand, Scarlett's love for Ashley is always an illusion, as is her idea that he returns her love and has married Melanie unwillingly. No matter how many times Ashley gently rebuffs her when she throws herself at him, she can't imagine that he would really prefer Melanie to her. It takes Melanie's death to teach her the truth. In contrast, Ralph and Meggie's love for each other is real, if forbidden.
This cannot be five thousand words, but it does offer several contrasts for you to further explore. The question is interesting, and you could easily go back to both novels and pull quotes that, for example, contrast both women's attitudes to marriage and life.