Who knows about which are seen as carrying more weight. However, I am sure that it is better to have letters from both sides. Letters from people with some authority are good, but really only if they in fact know both sides of the couple. In addition, the visa officer is concerned about whether the applicant is committing marriage fraud - the sponsor is not usually suspect; if it is a fake marriage, CIC usually thinks that the sponsor was duped. Occasionally the sponsor and applicant both know the marriage is fake and the sponsor is doing it for money or to help a family member. So letters from the applicant's side are needed, showing that the letter writer believes that the applicant is really in love with the sponsor. Letters from the sponsor's side saying he or she is a good person who wouldn't commit marriage fraud are less useful, since CIC assumes that in most cases of marriage fraud, it is the sponsor who is being tricked.
To sum up, ideally you will have support letters from both sides.