I think you mean just Jane in this question: throughout the novel it is clear that Lydia is flighty and headstrong and so has little to recommend her apart from her beauty and sense of fun. I guess your question identifies the bad aspects of Elizabeth Bennet's character. She has many great qualities - her beauty, intelligence and her wit, which is particularly important in a novel that is characterised by dialogue. In addition to this she is honest, which enables her to rise above the pettiness of the society of which she is a part. Yet, for all of these admirable qualities, we quickly see, especially in her interactions with Darcy, that she has a sharp tongue and a tendency to make hasty judgements (for she is the Prejudice of the title, just as much as Darcy is the Pride). These qualities are what the action of the novel helps Elizabeth to realise about herself and we can see that she is a better person, more self-aware, at the end of the novel.
In comparison, Jane seems to be very different to her sister, and almost is used to contrast her sister's fiery nature with her sweet-mannered and mild nature. She has none of her sister's failings and therefore is a much safer bet for marriage - but one can't help wondering if marriage to Jane would be a bit boring. She seems so perfect!