They are both effective in preventing the flu and are safe. There is some evidence that the spray (live attenuated influenza vaccine or LAIV) may produce a greater immune response in children than adults however as I stated they both protect one versus disease due to influenza virus infection. The spray is a live viral vaccine however it is attenuated or weakened so it does not cause disease and it is cold-adapted which means it cannot replicate at body temperature. Therefore replication is limited to the mucosa (lining) of the nose and cannot spread down the respiratory tract (e.g., the lungs). There have been no reported cases in which the nasal spray has caused influenza disease in a recipient. The "shot" is an inactivated small portion of the virus and thus has absolutely no potential of causing influenza in the recipient. Side affects of both types of vaccination are minor and transient, including tenderness at the site of injection, body aches, headache, and low grade fever with the shot and nasal stuffiness and fever in children with the nasal spray and sore throat with the spray in adults. But as I stated they are of short duration (generally 1 to 2 days at most). If these affects concern you I find that taking acetominophin or ibuprofen prior to getting the vaccine and for a day or two afterwards often alleviates these effects. Try to avoid aspirin during influenza season as there is an association between aspirin, influenza infection, and a condition known as Reye's syndrome.