I prefer the 20lbs running line when I am throwing very light bellies. Some summer stuff is so light, the mass of the 30lbs will stop it dead in its tracks unless you hold all the running line off the water. I hold loops but that doesn't mean there isn't a bit of running line sticking and that can be enough.
This is when using bellies under 200 grains in specific situations. Otherwise, I stick with 30lbs but I am preferring Rio Powerflex at the moment.