Well, like the name suggest: One has the Vario tech, the other one does not. What is Vario?
Here you go
So basically you can adjust the shape of the cooler to better fit the shape of the CPU IHS.
As for nickel vs silver - just go with the nickel one. It will give you 99% of the performance of the silver one.
IMHO: Just buy a nickel version without the vario and if you after the very best cooling performance, delid your cpu, apply liquid metal and smoothen the surface of both the IHS and the cooler by lapping it (-> basically just very fine sandpaper). This will grant you the very best performance, at a low price.