To be honest, Pekka, the zinc alloy would probably have higher properties and be better for the purpose than cast iron. Check the properties for za-12, including tensile strength and damping. If I cast it in iron, it would be more a matter of using my big furnace and melting iron once again for the fun of it -- and coming full circle in using so many different fabrication methods and materials on this one project.
The problem with most commercial die cast zinc alloy products is that the material and method were chosen to minimize cost, by using very thin sections and high speed production methods. Zinc alloys are so strong and easiy injection molded into thin webs that such are the norm. It's possible to find machine tools with 2 mm sections and complex webbing in ZA because it can be formed and used this way. You'd never be able to do that in iron, it doesn't have the tensile strength or easy injection molding capability.
ZA cost saving is not a priority in a home-shop built small lathe -- I'm only making one, and it's easier to just cast a thick section than a thin web. The cross slide on my lathe is 25mm thick ZA by comparison. It's stronger, stiffer, and more vibration absorptive than an equivalent thickness cast iron, part -- both are overkill, however, because of the need to be able to bolt accessories to the table through either Tee slots or tapped holes. So it doesn't really matter from a design standpoint, the thickness is determined by practical fastener depth requirements.
For the headstock, I'd say ZA has the edge, because of tensile strength requirements around the area of the bearing housings, and in general torsion on the whole at the center height for cutting and the pull of the drive belts, plus vibration damping qualities, re chatter. I'm sure both will produce good headstocks if properly sectioned, but if i were just to stack up materials properties for the same headstock sections, I'd say ZA would be better.