Could I chime in and mention that persistent cookies are perhaps more insecure on shared computers. i.e. you use a computer in the library, and afterwards your persistent cookie will remain on that computer, versus a session cookie which would be removed when the browser is closed down or the computer shut down.
If you forgot to logout of a website on a shared computer, potentially with a persistent cookie you would still remain permanently logged in when the next person uses the computer, unless the site in question queries last activity time and auto logs you out. Yes, this could still apply to session cookies if the browser isn't closed down, but by their very nature there's more scope for a session cookie to be destroyed fairly quickly (session timeout, browser closed down, computer shut down, etc) than there is a persistent cookie.
