Actually the big difference between the them is the electronic torque and the free speed.
ES120: 2.8kgf.cm and 270rpm using a 1:144 gear box reduction
ES121: 1.5kgf.cm and 380rpm using a 1:64 gear box reduction
Everything else seems to be identical, both have a maximum rated manual torque of 30kgf.cm and use the same 25,000rpm motor. Like you said, but their electric drive torque numbers being the same made no sense. I went to the Alibaba and bang good to get the numbers from the 'updated' versions of both. The ES120 update is likely to do with a new bearing setup for the locking clutch.