I'm trying to get a feel for how Storm's cover relate to each other regarding strength.

I know that Reactor is now used in the Tropical series balls and the Ride.  Per Storm's FAQ's R2S was a stronger version of Reactor and R2X was a stronger version of R2S.
R2Sâ„¢
    This coverstock was a chemically-enhanced material that originated from one our most famous coverstocks, Reactorâ„¢. It was introduced originally in the Special Agentâ„¢ in 2007.
R2Xâ„¢
    First introduced in the Gravity Shiftâ„¢ in February of 2008, the R2Xâ„¢ coverstock was derived from the R2Sâ„¢ cover but with new chemical additives that created more friction.

Where does ERG fit in the mix? Is that stronger than R2X? If you had a Crux and Crux pearl, what would be a good dropdown from there?