According to the Bekenstein bound, the maximum entropy in a given region of space is proportional to the area of a surface containing that space, not its volume so to really future-proof it, we should probably go with per mm^2...
Maybe for memory, but I think it's too early for logic where heat transfer forces you to keep volume per surface area under control. Legitimate 3D logic will require a radically parallel compute model, probably something neural, and until we see what form that takes I think it's better to keep the metric 2D.