The anticlunk spring absorbs the shock through spring action, lessening the beating that the aluminum case takes.
The case saver is a Last ditch fix for a case that is pretty shot in that area.
I have used the case saver piece to avoid a failure on a beat up case, and had no issues with it - still running a couple years later.. It distributes the shock load over a lerger area and onto part of the case that is still solid, not about to break off. However, being a solid steel piece it has no spring-like shock absorbing qualities.
If your case is beat, showing wear enough to make you think about using a case saver then its probably a good bet you should use one.