The advantage of slowing down water is that it creates an ecosystem. When I was young, the big push was to preserve wetlands, not drain them. Wetlands, by definition, slow down water. Water rushing down a waterfall does produce much of an ecosystem, though it is very pretty. (to use an extreme)
My opinion is what is needed is diversity and lots of it. If there are no wetlands, you won't have waterfowl and all the things associated with them. If all you have is still water, you may well not have trout and other species who need oxygen, etc.
Perhaps the biggest argument against online ponds, is that unless you want to be as busy as a beaver repairing it all the time, it is better to be offline.
Seasonal fluctuations of water flow can be very hard on a pond.
The problem is probably placement. For example, to do as
Sepp Holtzer did, which was to make a wetlands deeper is nothing more than turning back the clock of time since it originally was a pond most likely that filled in. But, to dam a fast moving stream creates something was not there, and won't be there in the future - so you have forced your will on the environment in a way that isn't very appropriate, and may well result in degradation of the system.
I have an online pond that is where two spring fed streams (originating on my property) come together in a bowl. In the past, this was just wet, not even a decent wetland and I had to build a bridge there. So, instead, I built to large ponds which are now full of life and the water is cleaner for them since I planted
trees around it, and of course, the shade keeps the water cooler. And, instead of being shallow stagnant water, it is now eight feet deep, which results in cooler water, not warmer. The total cost for the system was 5,000 USD, so it will stand up to fluctuations. And yes, my ponds have tropical waterfowl, a small species of caiman, fish, etc .
So, all of that to say this, it isn't as simple as offline vs online.
