Another factor in favor of the GW plastics is that they can be posed and otherwise modified very easily. Because they are a collection of bits and pieces you can use pieces from different sets together. For example when I needed a hunchback I combined the legs of a zombie, the torso of a night goblin, the head of a chaos warrior (the one sans helmet), and the arms of an Empire swordsman, adding a zombie slung over his shoulder for fun. I also like the ability to shove a box of pieces at my players and tell them to make their mini.
The Mordeim set of miniatures even includes a frame of backpacks, lanterns, books, and other adventuring gear.
The Auld Grump