Hindsite is expensive! Do you have a boiler? You have a lot of mass to heat up. Electric would have worked in the floor sandwich (i.e., under the tile), but I don't think anyone would recommend it as a staple up. Hydronic staple up would work, but your supply temperature will need to be much higher (and thus less efficient) plus, you'd need plate diffusers to spread the heat more evenly. Response rates will be very slow. Either would have been much more efficient and easier to install as part of the mud bed underneath the tile.