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It's best to use E = (1/2)mv^2 because you're given the mass m of an electron and you can just solve for v. Also, it doesn't matter what the electron is being described as since the equations you use depend on the information provided to you.
What information must be provided to determine which equation to use? Is it that E=pc can only be applied to a massless particle because the speed of light is for a massless particle/photon?
E=0.5mv^2 is the generic physics equation for kinetic energy. It really depends on what variables you are already given when you decide which formula to use. When discussing the photoelectric effect, it is more common to use the Kinetic energy equation.