Frederik wrote: "Well, for my level of mathematical rigour (ahum) those two options are more or less the same .."

Depends on what you mean with more or less:

$Log(GDP(year) - log(GDP(year-1))= log(GDP(year)/GDP(year-1)) = log(((GDP(year)-GDP(year-1))/GDP(year-1)) +1) = \Sum_{n=0}^\infty Log^(n)(1)/n!(GDP(year)/GDP(year-1)-1)^n =$ (assuming that Log is Ln)$ = 0 + (GDP(year)-GDP(year-1))/GDP(year-1) - 1/2 \cdot ((GDP(year)-GDP(year-1))/GDP(year-1))^2 +.....$

@rks: I think the rise in productivity is also to a great extend due to substituting labour by machines, which also needs extra energy.