Two factors that affect how well a rotor will cool - Conduction and
Conduction is the transfer of heat within the rotor. If you can cool the
surface, but the heat energy can't get from the center to the surface,
your rotor will keep the energy and become a cause of fade. The thermal
conductivity of Aluminum is between four and ten times that of Steel at
room temperature (300K). As temperature increases, the gap widens - Al
performs better in this regard at higher temps than lower, Steel doesn't
change much. Trivia - this is called the thermal conductivity.
Convection is how the heat is tranferred away from a surface (here the
rotor) by a fluid (here the air). Factors are the Convection Heat
Transfer Coefficient, surface area and temperature difference between the
surface and the fluid. If the surface area and temperature difference are
equal, only the Conv. Coefficient matters. But the Conv. Coefficient
doesn't change with material. So heat is transferred away from the
surface of the Al and Steel rotors at the same rate under identical sizes
and temperature differences.
My research indicates that the Al rotor will dissipate more heat energy in
a given time period. The energy if the center of the Al rotor can reach
the surface faster than the Steel rotor. When the steel rotor has cooled,
it has to wait longer for the heat energy in the center of the rotor to
come to the surface. However, the Al rotor moves the heat energy to the
surface much faster, giving a higher average temp for the surface, giving
greater convective cooling.
I hope this helps.
FWIW, the relationship between convective heat transfer to conductive heat
transfer is the Biot number (I had my heat transfer book out for the rest
of this discussion, thought I'd pass it on).