Family member vs stranger as surrogate

We are starting to research family building via surrogacy. My husband and I have been TTC for about 5 years now, fertility treatment for 3 years. When we first found out we would have trouble conceiving, my cousin said she would be a surrogate. It hasn't really come up since then. I am sure there are pros and cons to having family member carry our baby or someone we have never met.

Right now I think it would be emotionally easier on me to have someone I have never met carry our baby. I don't know how serious my cousin was about it or if her feelings would be hurt if we went through an agency to find surrogate.