As far as anyone knows, the guts on the insides are the same.
The Premium have a slightly less grip around the head then the Pros.
My DT990 Pros feel just fine on my head.
The Premiums come in 32-Ohm and 600-Ohm, which the Pros do not.
The 32-Ohm and 600-Ohm versions are "special" so they cost more.