So I should get the bigger one.... Bigger is better.... Im setting up a 20l but more than be likely be upgrading it to 40b.... Right now I have a 125 with 2 sera flora reactors.... Not too impressed with them.... The 20l is gonna be an experimenting tank
You certainly cant go wrong with larger. However its when you have a small nano sized tank that you dont want to go bigger.
If we were doing a 125 ideally you want an inline diffuser. But if you want to use in tank model diffuser I would advise on using a manifold on your regulator so you could work 2 diffusers at one time. Mist is king!