This week's book giveaway is in the OCPJP forum. We're giving away four copies of OCA/OCP Java SE 7 Programmer I & II Study Guide and have Kathy Sierra & Bert Bates on-line! See this thread for details.
validation.xml comes with struts validator framework. It gives you certain set of validation rules, which you can use conveniently from xml files. But with ActionForm validate method you need to write your validation rules programmatically. So I think validator framework makes it more convenient for you.