Difference between E-Street and RPM Heads ?

What is the difference between the Edelbrock E-Street and RPM heads?
This is for a Big Block, and looking at the 75cc versions of each.
It looks like they both use the #5792 Valve springs.
Do they use different quality valves?
Not sure why the RPM is $556 more expensive per pair?

Powdered metal vs ductile iron seats, and slightly different valves.

That’s it.

The minuscule amount of blending done on an rpm doesn’t do anything.