I prefer: /Comments/Search/3?sort=author. The querystring is a good place to pass in programmatic parameters, especially if the parameter (like in this case) is not important for SEO purposes. If the parameter had some semantic meaning as a search term, the first URL would be better.
In a controller method you can use something like this:
public ActionResult Search(int id, string sort)
ASP.NET MVC will automatically wire up querystring values to the parameters of your method.
Use the following route
routes.MapRoute(
"Default", // Route name
"{controller}/{action}/{id}", // URL with parameters
new { controller = "Comments", action = "Search", id = "" } // Parameter defaults
);
/Comments/Search/3?sort=author will call Search(3, "author")
/Comments/Search/3 will call Search(3, null)
Keep in mind that id is mandatory so this url will fail:
/Comments/Search