Is the voltage difference potential or potential energy?

Electric potential energy is the energy it takes to put a charge in a given place, for example, near another charge.  If the particle has to be held back (because it is attracted) then the potential energy is negative; that's because you can get energy from that particle as you move it in.

Electric potential of a charged particle is the potential energy divided by the electric charge of that particle.

The sample with the lowest specific heat capacity will have the greatest temperature increase. in this case, it would be platinum.