I guess I don't do enough roofs to have a swivel on my roof cleaning hose either. At low pressure it doesn't get tangled the way the high pressure hoses will...
I run 5/8" hose. My dilemma was with guys talking about their super swivel lasting years and years, I was considering the 3/4" in case I decide to upgrade to an air system.
I'm always trying to build/do things with the future in mind.
In bleach service, hard to find anything that lasts years except the hose and air pump. I would go with the 1/2" inch swivel as long as the manifold and hose are the same size, if you upgrade to an air diaphragm pump, the 1/2" hose will still spray a lot, probably enough to cause runoff issues which will be another problem by itself.
If my understanding of their sizing chart is correct, there's a difference of a little more than 1/16".
I'm going to give them a call to ask about the flow rates.