Quite interestingly, the two implementations track each other fairly well. Both have around a 30Hz per mA or 30uA per Hz response.
Also very interesting to note is that the response is not linear. The higher the frequency, the more current is needed for the same change in Hertz.
I measured the 3dB point at 16 selected currents:
The CV is the input to a constant current circuit. It is the voltage across a 3k3 resistor, and the resulting current (buffered to stay constant) is split between the four cells of the filters.
I selected 8 tightly spaced voltages and 8 further apart, as the lower end requires less change to double the frequency. Deltas are the difference between two measurements. F_c is the 3dB cutoff frequency point. This is a linear CV, e.g. without the exponential converter connected.
The result ended up like this:
So what does this mean? It means that the V/oct tracking of the filter is not perfect. It will be slightly more closed as one moves up the scale on the keyboard. I have read about this effect earlier, and the writer noted that it didn't seem to matter all that much. I guess a little difference is hard to hear when the base frequency changes a lot.