.NET AJAX Calls to ASMX or ASPX or ASHX?

What is the most efficient way of calling some business logic from javascript on the client side using AJAX? It looks like you can call a [WebMethod] on an aspx directly from javascript (in my case I'm using JQuery to help out) OR you can call a .asmx directly. Which call incurs less overhead? What is the best practice?

Also, what does the [ScriptService] attribute do on a class? I have never used this before on my .aspx [WebMethod] methods and everything seems to be working fine.

I'm hoping this is a purely objective question. Thanks in advance!