Its interesting to see the different route that they've taken with the two sprues. The Perry Brothers sprues are made by RENEDRA which is run by a veteran/s in the plastic manufacture biz. The plastic they use looks similar if not identical to what GW uses. It carves well and bends, a little, before it snaps. And will bounce when it hits the ground.
Valiant have used a much stiffer material more similar to the older Hasegawa or Prieser figures. They are able to make really fine pieces with this material it seems. You only have to look at the shovel handles on the backpacks or the mortar assembly to see what they've achieved. The drawback is it's brittle, for example i lost the muzzle off the standing Thompson gunners smg when i dropped him on the table during construction and the mortar bipods leg broke in half as i was cutting it from the sprue. Plastic Soldier Review have an interesting article on these figures as well.
