jquery+jstree in c#.net - webservice responseformat?

Question

I have been playing with jstree (1.0rc2)+jquery (1.4.2) for the first time with c#.net and although I have gotten it working, there are a couple things that I don't understand about how data is provided to the tree by the webservice I use to populate the tree (using ajax and the json_data plug-in). I was hoping someone with more experience using jstree could provide some insight.

The jstree config looks like this:

 "json_data": {
                "ajax": {
                    "url": "GetTree.asmx/GetChildren",
                    "type": "POST",
                    "contentType": "application/json; charset=utf-8",
                    "dataType": "json",
                    "data": function(n) {
                        var result = "{'id':'" + (n.attr ? n.attr("id").replace("node_", "") : "0") + "'}";
                        return (result);
                    }
                }
            }

GetTree.asmx GetChildren method:

   [WebMethod]
    [ScriptMethod(ResponseFormat = ResponseFormat.Xml )]
    public string GetChildren(string id)
    {
        List<jsTreeNode> jsTree = new List<jsTreeNode>();
        //... (build the tree as needed)

        JavaScriptSerializer serializer = new JavaScriptSerializer();
        return(serializer.Serialize(jsTree)); 
    }

Question 1: So everything works great, so what's the problem? The problem is "ResponseFormat = ResponseFormat.Xml". I struggled for a while to get this working because it did not work when it was set to ResponseFormat.Json, which is what I would expect it to be. In that situation, no errors would be reported by the web service or by jQuery when parsing the json response, but the tree would render empty.

In looking at the HTML output of the web service, I could see no difference between what was rendered either way. I was hoping someone could explain why this works (counterintuitively) and why it does not work with ResponseFormat.Json, and if this is indicative of something else I might be doing wrong.

Question 2: Generally, web service or web handler?

Would using a generic web handler (ashx) be a more efficient way to do this anyway? Is there a substantial difference in the overhead required of a standard web service versus a generic web handler? Since my goal is basically to control exactly what is output (and using the json data format in the web service doesn't seem to be working the way I want it to anyway) I am not sure what benefit, if any, there is to using a web service here instead of just stripping it down completely. On the other hand this works now so maybe I should leave well enough alone.

Thanks for any insight you can provide.