It's just a name
They didn't used to be all that quiet, the new models may not be too bad but it is just a brand name. I don't have any experience with them, so I don't know how they compare to others. The old models also used to have reliability issues. They were external pumps made of ceramic and they were prone to heat stress cracking the units. The current models are not like that, however, totally different design. They are probably fine pumps.
Anyhow, no - you don't want 2 times turnover. Sorry, I meant whole system turnover. So basically volume of water in the tank, plus volume of water in the sump, minus whatever for rock and so on. With a 72g tank, 20g sump (running at probably half way full roughly - more on this later), you're probably looking at around 80 gals total volume, you want 5-7 times THIS volume per hour (so 480 gallons per hour). Further, you have to look at the pump curve data and figure out what gph it pushes around 4-6 feet of head pressure. So you're probably wanting a pump that's rated for 700gph at 0' head which is going to translate to (hopefully) around 500gph realistically for a sump return.
You can go higher of course, but two things to take into account: the higher the flow, the higher the water level in the tank above the overflow line. And the higher the flow, the faster the water travels in the sump. If air bubbles from the overflow make it to the pump intake, then you'll have persistent microbubbles. So that's why I prefer to not oversize the sump return pump.
But you do want a minimum flowrate so that if you keep a heater in the sump, the heat from that heater is making it to the main tank.
Hope this is making sense, sorry if it's overwhelming.
PS. Forgot the "more about this later" part. Make sure that whatever volume of water in the main tank that is above the overflow and any kind of anti-siphon break in the return plumbing can fit into the sump in the event of a power fail or pump shutoff. If you run the 20g sump at 1/2 way full then you have room for 10g (which is probably plenty). I'm sure you know this but just thought I'd be safe and mention it nonetheless.
