I have been using the sweetwater fine stones from www.Aquaticeco.com
These are as good as the Azoo disc if not better
and other disc, quite consistent also.
They cost 1.70$.
They are better than the Rena stuff.
Hard to beat.
Hardly any back pressure too.
I also used the ultra fine diffusers, the plate type that need 25psi to run them. These are super, but suitable for large tanks(55$).
Venturi designs are very good and produce the best mist really(many SW folks will attest). I have a needle wheel powerhead that is specific for venturi type skimmers and it's a super way also to add CO2 to a FW tank.
I would like to get the diffusers on all types out of the tank and blasted into the outflow of a pump/caniters, return etc.
The mist blasted around a tankl works better than any methods I've seen, and I've been playing with CO2 for a long time.
You can see the effect.
Folks can carry on about why it work all day, but the fact of the matter is that it does indeed work better.