8kt.snr.fs
FOM1 is the most quoted FOM, however, FOM2 is better suited for thermal noise dominated ADCs like pipelined ADC.
FOM is the energy efficiency. In thermal noise dominated ADC, the low power consumption limit is P = 8KT. Fs. SNR^2
So for FOM = P/(Fs . SNR^2) thats why we have FOM2.
Also using FOM1, low resolution looks better. Whereas, using FOM2 give credit to higher resolution.
They are different because of the precision of 2^2ENOB. and their units are also different.
In the papers you will see they use FOM1. So you can use that FOM1 without any concern. However, I am more inclined towards FOM2 because of the reasons I mentioned above.