Summary: use the Client Side Usage (Javascript) for any technology (Web Forms, ASP.NET AJAX, MVC, HTML, ASP). Use the Server Controls usage for Web Forms only if Client SIde Usage is not preferred.
In RapidSpell Web up to and including version 3, everything was Server Control based (ie. ASP.NET server tags), nothing was Javascript based. During this period of time other technologies have been released, such as ASP.NET AJAX, JQuery, MVC, and MVC Razor - the trend has been for ASP.NET developers to be more Javascript aware. With multiple 'form factors' in use, it makes sense for RapidSpell Web v4 to become Javascript based, which has advantages;
The Javascript method and Server Controls are incompatible and must not both be used on the same page.