Miliput is, as pickle put it, better for larger stuff. Green stuff is also known as Kneadatite. Well, i would say it is a very good material for 'base' work, not bases as in those your model stands on, but base as in the start of a fully sculpted model. Milliput costs less (while Greenstuff costs lots and GW GS cost a STUPID amount) and sets VERY hard,it will be easy to start a model by making the basic shape in Milliput and then sculpting the details onto with greenstuff or brownstuff dependant on the texture and properties you require from the materials once cured.
If you can get hold of milliput for some reason, do it on the cheap, use plastacine and superglue, its messy and doesnt do the job as good but its fast and lazy!
The previous users said almost everything. What I know from my past use of both materials that GS dries and is strong. On the other hand if you use Miliput for detail work on a mini and let it dry there is a chance that it will break of a misuse, as if you hit the bag or you drop the mini - by mistake always!.
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