Major benifits of the ZS over the Audigy is they are common, slightly better better audio quality and the final card to support Win9x.
Would I bother upgrading if I already had a Audigy 1? nope.
Benefits over the Live!, bigger difference in audio quality but I'd still be hard pressed to know the difference, Audigy supports EAX4 vs EAX3 of the live but that doesnt matter in Win9x , Optical out if that matters, the big difference IMHO is the Audigy drivers seem less bloated.
Would I bother upgrading? Yes but only casue it makes me feel good knowing I've I sweet card in my retro rig. I've got a Live in another PC and if I'm honest I probably don't really notice the difference.
Sound will always be subjective, some will say they notice night and day difference and someone else (me) won't.
I'd say if you notice a difference between the Live! and the Audigy maybe you'll notice it between the Audigy and Audigy2, maybe