Sorry for the late reply Arkoudaki.
Basically it works out like this, and this is just my own personal experiance, other sculptors may use different techniques that give them different results to mine... but here we go:
Milliput has a pretty chalky or crumbly nature that can sometimes make smaller details hard to make or weak once its hardned.
Green Stuff has a sort of rubbery texture that makes it hard to sand once its cured, but it is strong and a little flexible.
Once mixed, the combo holds detail well, is strong, and can also be carved and sanded easily.
Hope this helps.
-Kosta