I am running a eheim 2026 on my 20 high. It was recommended on this forum that I do it. I thought for sure that it would be too much, but i tried it. Nope, I would say just enough flow.
The fluval 104 doesn't come with a spray bar.
It comes with a diffuser. For my setup, I didn't want to use the green eheim tubes, so i switched to the input/output from my magnum 350. It has a similar diffuser. You can always switch to a spraybar if desired.
The fluval 104 should be good. You could even go larger. The max tank size listed for the 104 is 25 gallons. Folks recommened to me (I have the 20 g high too) that i go with the XP2, the 2026, etc...which i say would compare to the 204. A canister opens up the possibilities for adding various inline equipment such as heater, uv sterilizer, co2 reactor, etc.
If I were you, I would keep your CO2 setup the same for the time being. You might want to add an additional bottle to your ladder or go with a larger bottle, but a glass diffuser really doesn't work great on a diy yeast system. I just got done talking to a guy at a lfs that was showing me his nice blown glass diffusers. The bubbles produced by the yeast method can eventually clog them, etc.
Anyway, I wouldn't start changing your whole setup until you really get a feel for your current setup.
Just my 2 cents.