PoC could be still more energy-efficient. I didn't investigate in depth, made only some general assumptions (about energy use in hard drive manufacturing):

- attack cost (achieving 50% of the validation power, e.g. hashrate) determines the security of the chain. Thus, to become equivalent of Bitcoin security, all PoC farms/botnets together would have to be so large that the attacker needs the same amount of money to achieve 50% than he would need to 51% attack Bitcoin.
- PoW mining cost is not much higher than energy cost (hardware costs are only a little fraction)
- PoC mining cost, in contrast, is more tied to hardware cost
- energy consumption makes up a relatively small fraction of hardware manufacturing cost (certainly less than 50%, probably less than 25%)

Thus, we could assume that PoC, for the same "attack cost" level, needs, at most, 50% (probably 25%) of the energy of PoW mining, but on the other hand needs more other resources. Because of that reason, I don't know which one is "better for the environment".

(I know Paul Sztorc's article, but I don't consider it convincing, because not all human labor damages the environment.)