I've always taken the title to refer to two main characters - Elinor (Sense) and Marianne (Sensibility). I've never thought of Marianne as the sole main character for this novel (and if I had to choose only one - it would be Elinor anyway).

The narrator tells us here that Marianne has some sense - I've always taken that to mean that she could exercise that if she wanted to, but she prefers going with sensibility instead. She refuses to be polite to Fanny and tell Edward exactly what she thinks of his reading ability. She forgoes sense to focus on sensiblity.