Is it any less efficient to drive a 12V version of a heat cable from a step-down buck than directly powering a 230V version of it directly from the wall?
Of course 12V is less efficient because of convrter loss. But you have to consider the safety of two different systems. If you want to heat your bed, I would reccomend you to use a 12V vesion, but no buck, rather and isolated PSU. If you will heating a concrete floor, then a 230VAC is more appropiate, however a GFCI device might be needed.