Elinor is often thought of as being entirely composed and sensible all the time, I think, but the passage you point out clearly indicates that she is not. She may be calm and collected but underneath her self-control, Elinor does feel deeply and carries a lot of responsibility on her shoulders for a person of her age.

In contrast, Marianne is outspoken, vivacious and fun-loving, yet in an innocent way, and her "abilities were, in many respects, quite equal to Elinor's", as the passage you quote says. Those three words, "outspoken", "vivacious" and "fun-loving", could be applied to Lydia in P&P (are comparisons between JA novels allowed in GRs, BTW, or should I avoid stepping into Austenuations territory?) but Marianne's personality is vastly different from Lydia's. Marianne has far more gentle innocence and sincerity than Lydia.

One thing I always ponder over and think about when reading S&S is whether or not JA thought that showing one's feelings in a candid way is a good personality trait or whether JA preferred feelings to be guarded with self-control. For instance, JA seems to express that Marianne should control her strong emotions more and sometimes JA also seems to have more regard for Elinor's temperament, in part.