Well, jQuery is JavaScript, so it's not really one or the other. But there are distinct advantages of using jQuery over writing raw JavaScript. These include, but are not limited to:
The amount of code you need to write is vastly smaller
You just asked if you could jQuery in place of JavaScript. The answer is no, because jQuery *is* JavaScript. You can use jQuery instead of the default JavaScript/browser support for Ajax (and should, since it's substantially easier, and eliminates the cross-browser headaches).