1) assigning a PEC material to a 3d object, then giving all faces of the object finite conductivity boundary condition using copper parameters
2) assigning copper material to a 3d object.
Are they interchangable? I ask because with PEC material i wouldn't have to solve inside (reducing simulation time). And perhaps the copper finite conductiity boundary will take care of skin effect?
I don't agree
You are modeling two different problems
3D PEC+Cu Sheet Only _surface_ currents are computed _over_ the Cu sheet
3D Cu model + solve inside _Volumetric_ currents are calculated _inside_ the model
Why are you going to obtain the same numerical results? Because at high frequencies the skin effect is very important and the currents are restricted to lay on the conductor's surface.
The lower the frequency the different the results.