I am not an expert in this, and am still learning and experimenting, but the "ADA-knockoff" ceramic disc
diffusers sold on eBay by AquaticMagic have worked pretty well in my 20-long (30in).
Also, putting the diffusor right below or next to the filter intake seems to help distribute the bubbles. I have not done a scientific test of this and it sometimes causes my Rena Filstar XP1 to "burp" occasionally, but the spray bar seems to throw the bubbles all over.
The "ladder" type did not do anything for the tank, just seemed to waste CO2...
I have been trying to make one of the inline reactors out of PVC to experiment with, but my construction skills are still nascent.