Arsten, YES! I totally hate the term "Cersei of the Starks," but it's more of a concept than an actual thing.
I also agree with you completely that Sansa gets a bum rap because she is so often compared to Arya. Your point that Arya is the exception and not the rule is also very correct. Arya never had any designs on being a lady and thus was not interested in "lady things." She was more comfortable with a sword in her hand than needlework. The other thing to keep in mind is the different mindset between the two, not just their temperaments.
As Catelyn ruminates, Arya is "Ned's daughter." She's a true Stark Maiden of the north, compared often to her deceased Aunt Lyanna, who was known as both a great beauty and very wild. Lyanna, as we know through Meera's tale, was more than capable of holding her own with the boys.
Sansa on the other hand, she's a mama's girl. She takes after Catelyn. Sansa, initially, does not worship the Old Gods but the New. She worships the Seven in the sept built by her father for her mother. Sansa, being a mama's girl, also adheres, initially, to the Tully words "Family, Duty, Honor," and wants very much to emulate Catelyn, whom she holds as the ideal lady (until she meets Cersei, before things go badly in KL).
Sansa's naivety is in large part Ned's fault. So what that they were in the North, cut off from the rest of the Westeros? Ned knew that, being so high ranking a noble family that chances were very high Sansa would be married to someone in the south. While perhaps not able to directly expose her to more southern culture, he should have at least educated her. Additionally, Catelyn notices Sansa's rather whimsical approach to life and literally does nothing to educate her daughter that life isn't remotely close to what the poets make it out to be. Both of Sansa's parents are guilty of wanting their child to remain a child, knowing full well that Sansa, even more so than Arya, being their first daughter, would ultimately end up somewhere in the south, where she would not be sheltered. They both should have sat down with her and had a conversation with her somewhere along the line, especially Ned in KL. Telling her "Oh, it's dangerous here," without any further information? That's the equivalent of telling your kid "The boogeyman is out there, don't go out," which any intelligent child isn't going to buy. If he had, for even a moment, invested the time in treating Sansa as an intelligent child, well...
Going back to Arya, her being a daughter of the North, her insights are keener than Sansa's. We also know that Arya is far more interested with things that are, not things that could be. Arya's grown up on the same songs as Sansa and her northern blood knows it's all crap. Sansa is much more the stereotypical little girl who believes in fairies, unicorns, rainbows and true love. She was an optimist until reality quite literally beat it out of her.
I used to dislike Sansa in the books, but I so often found myself playing devil's advocate to those who really hated her, I was eventually bought over by my own arguments. Until AFFC, Sansa is a poor, sad, abused girl who loses everything she holds dear. Finally, once safely out of Lannister reach in the Eyrie, she's able to start applying the brutal lessons learned at Court.
She may have started out whimsical and naive, but experience has transformed her. She started out yearning for the South, but her Northern blood has quickened within her, and she's finding strength and power she never knew she had.
