Right... so one of the main differences between WebAssist Server validation and spry validation is that server validation takes place on the server after the form submits and spry validation uses javascript in the browser. Someone can disable javascript and bypass spry validation at any time and server validation is much more secure.
Also spry validation works on a set structure that you need to use for it to work in terms of layer names and style references you need to use. Validation toolkit has you do all of the design and works with what you design so it has no limitations on how it can manifest itself in terms of design.