I read this question and implemented the approach that has been stated regarding setting the response HTTP status code to 278 in order to avoid the browser transparently handling the redirects. Even though this worked, I was a little dissatisfied as it is a bit of a hack.

After more digging around, I ditched this approach and used JSON. In this case, all responses to AJAX requests have the status code 200 and the body of the response contains a JSON object that is constructed on the server. The JavaScript on the client can then use the JSON object to decide what it needs to do.

I had a similar problem to yours. I perform an AJAX request that has 2 possible responses: one that redirects the browser to a new page and one that replaces an existing HTML form on the current page with a new one. The jQuery code to do this looks something like:

$.ajax({
    type: "POST",
    url: reqUrl,
    data: reqBody,
    dataType: "json",
    success: function(data, textStatus) {
        if (data.redirect) {
            // data.redirect contains the string URL to redirect to
            window.location.href = data.redirect;
        } else {
            // data.form contains the HTML for the replacement form
            $("#myform").replaceWith(data.form);
        }
    }
});

The JSON object "data" is constructed on the server to have 2 members: data.redirect and data.form. I found this approach to be much better.