I am interested in getting a electro etching unit. Tracy sells both these units. In your opinion, do I need variable voltage control offered by the Plus? I will ultimately buy a good unit, but if I don't need the control I'd like to save money for materials. The only thing I have interest in etching is knife blades made from stainless steel (ats-34, 154 cm) and carbon steel (o1, a2, d2). Thank you for your input.