Does the material matter? Usually they are made of copper wire, but is there
any reason not to use spring steel?
For normal mode HF mobile whips, I'd expect the obvious - ohmic resistance - to
figure in there. As the radiation resistance of these is v.low the efficiency
will suffer as the conductivity goes down.
For axial mode types more usually found on VHF/UHF (where circumference
approximates a wavelength), the conductor material is largely selected for
mechanical properties. Stainless is quite common in commercial units.