Heat Cable: directly powered with 230V AC vs. 12V DC with step-down efficiency
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.