This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
"/Lookup" is preferred because it will work in all cases. "Lookup" will not work if your current context is other than the root. For example, if your current JSP was displayed by the action "/foo/bar/MyAction", and you specify "Lookup", Struts will be looking for the action "foo/bar/Loookup".