LOL, you could write a book on that. Some people say you only need about 4-6X turn over. On an 80 gallon tank that is 320-480 GPH. The reasoning is that because you have to battle against head pressure from the pump, you are wasting electricity and adding heat. So the powerheads are a much better solution for creating the water movement inside the tank. I personally like to run mine higher at about 10-15X turn over.
There are a few things to consider:
Firstly, by keeping a higher flow moving through the sump, you can keep detritus from settling in the sump. Some people will use powerheads for that, but they cost money (typically more than purchasing 1 larger return pump), and use electricity. Maybe not allot, but it all adds up. Plus you have many more pieces of equipment to maintain and clean.
Secondly, heat. Some would say that you are adding extra heat to the system with a larger return pump. That is true, but typically my tanks have always stayed within temp and the heaters cut on and off through out the day no matter the time of year. Even when running large MH bulbs, my heaters would still start coming on around 2-3 hours after the lights cutting off. I was always able to keep my tanks below 80 because we live in Texas and run AC almost all day everyday. So since 99% of electricity that goes into a submersible pump is transferred back to the water as heat, I just had heaters that ran less. It is still a watt of heat whether it comes from a pump or a heater. I would much rather have it be heat generated from water flow, as opposed to a heater. It is even less of an issue today with most people running LEDs.
Finally, noise. Larger pumps make more noise. Typically if you take two pumps of the same model or quality, the larger the pump the more noise it will make. Although many of today's premium DC and/or standard submersible pumps like the Sicce models are very quiet. If the overflow is designed properly with flow in mind, they are completely silent as well.
It really depends on what your needs and situation are. If you live up north where you don't have or use AC during the day where it can get warmer in your house, then you would probably want to avoid extra heat at all cost. If the tank is by your bed and any large pump is going to be more noisy than you can bear, then use a small pump. There really is no right answer, it is about preference. For me, my tank is always in my living room and we run a tank heater year round, so the extra flow is a plus. Without having to have redundant powerheads in the sump, plus the ability to process more water through a filter sock quicker all while skimming the water surface better, I feel higher flow works better for me in my situation. HTH
