LED's brightness difference in these 2 circuits
It would depend on the device used as the inverting gate.

As the answer states:


  The Low-level output current (20 mA) is much higher than High-level output current (1 mA)


One can deduce the inverting gate IC can sink 20mA to GND but can only source 1mA on its "high" state, thus limiting the current on the LED.

It is fairly common for chip to be able to sink more current than sourcing, due to transistor configuration.  

Overall this is totally depends of the IC chosen, and there is no part number given.