I have actually used that diffuser, it is great, the quality is great and the CO2 is like pollen, never seen anything preform similar. It is also one of the few glassware items that ADA has hand made in Japan still, major bonus.
It is also unique as I don't see any knock offs that have the curved glass tube this one does. Most copies are of the ADA beetle design.
For disc glass diffusers that you see in the tank, I always recommend ADA, they are pieces of art in themselves that combine form + function perfectly.
Since your tank is only 20g you can also look at the Pollen Glass Type 1, 2 & 3 which also would work and are slightly cheaper (half $60).