There was an argument on which is best to use. But theres nothing really to argue about..
Here are your things each are best for.
Sessions
- Good for high Security - making people have to login after either logging out or closing the browser
Cookies
-Low Security A good example is this Forum. Good to track when a user is online(As well as Sessions)
-Users dont need to login unless Cookie Expired
Take that into consideration. Think of the situation you need to store the users information(Never Store a Password).