How do you decide whether you need to go to a massage therapist or a chiropractor?

Varies greatly: This depends on what you are trying to treat and who your practitioner is. For instance craniosacral therapy is done by some some massage therapists and chiropractors - good for various musculoskeletal problems. Generally, chiropractors work on joints(ribs, spine...). Massage therapists work on the soft tissue (muscles, connective tissue...). Ideally both joints and soft tissue are treated!

Depends: They offer the same thing - relief by massage and no diagnosis or cures. Massage therapist are gentler, work in a more comfortable environment, and will not attack you with unshielded x-rays or bogus theories. I would pick the massage therapist myself.