At this time...
it sounds as though it doesn't matter that much right now--your flow is adequate for your filtration needs, and it sounds like you are happy with your surface agitation. I have two smaller Aquaclear units, on smaller aquaria, and I use the high-middle range most of the time. I feel like I want to get high levels of filtration and agitation because the tanks need it.
I think the distinction between using low and high is that on high you will process more water, and potentially more waste overall, while on low/recirc. you are filtering what water that does pass through the unit more efficiently. Something like that.
I have a suggestion that works well for me: I take pieces of thin polyfiber mats (aquarium quality) and put them at the bottom of the filter stack, under the foam sponge. I am using it as a pre-filter. I suppose that I could use a pre-filter on the intake, but I can't find a small, inconspicuous, cheap thing to do that. I have been using Fluval filter polishing pads, which I can tear into at least quarter thickness, sometimes thinner. I have messy tanks so I am able to block lots of organic slop which would otherwise get into the sponge and the bio-media. I change them about every 7-10 days.
Hint: make the mat thin and change it often. Too thick will slow things up too much, in my experience.
