Basically there are only 4 or 5 producers of clutches in the world: Valeo, Daikin/Exedy, XDR, etc... Z32 stockers are all Valeo, the Clutchmaster or DXD clutch you get will be a Valeo Clutch with HD components (disk material, hub springs, pressure plate springs) I have no clue why DXD is so much cheaper, I could guess it's because they do all assembly work in house instead of just mix and matching components (order springs, disks, disk material, pressure plate shells, etc. then throw a kit together). When I got my WRX clutch from them it came with a hardened Exedy stg2 disk, organic/non-organic HD disk material, HD springs and a beefy Exedy(i don't know what it is exactly) pressure plate basket.
Read about the differences between all the FX100-FX500 and on:
http://www.clutchmasters.com/index.php? ... oducts.tpl , that should give you a bit of an idea about the availability of clutches out there, they are built in the same manner, also check out this site to get the basics all the components of a clutch function:
http://www.howstuffworks.com/clutch.htm