There are six primary materials available that fall into two basic categories; styrenic and olefinic.

Material Trade Name	Material Full Name/Discription	Relative Cost - Basic Function
styrenes
EPS	Expanded Polystyrene	Low cost basic cushioning/blocking and bracing
R-MER®	Butadiene/Polystyrene Co-Polymer	Medium cost and somewhat resilient (more than EPS)
Arcel®	Polystyrene/Polyethylene Co-Polymer	Medium cost and somewhat resilient (more than EPS) and very durable (tough)
GECET®	Polystyrene Maleic Anhydride Co-Polymer	Medium cost and able to withstand higher temperatures (210 ~ 250F)
olefins
EPP	Expanded Polypropylene	Higher cost and very resilient - general purpose resilient material
EPE	Expanded Polyethylene	Higher cost and very resilient - soft and good shear strength too (very low wt. and high wt. applications)

The relative costs are only intended to be a helpful predictive index.  In fact, the higher cost resins don't always turn out to be that way... For example, if migrating into a higher cost resin avoids product damage or reduces the overall package size, the net cost of the pack could actually be less than that of a lower cost material.  In the end, each product, it's fragility and logistics profile dictate material choice.  It boils down to a cost/performance trade off.