Canisters are essentially a closed loop system. A sump will use an overflow of some sort in the main tank to drain into a sump tank under it and use a pump to return the water back up to the main tank. A sump can take far more planning in initial set up. But they are far more customizable, typically much easier to clean than a canister (takes me less than a minute to change out a filter sock about once a week), you can put equipment such as heaters in your sump rather than the main tank. Also many return pumps on the market these days are more efficient than canisters, and can also give you a higher turnover rate for larger tanks. The FX6 and 2262 are two of the larger cans on the market. They both are rated around 900 gph at the pump. After head pressure and whatnot, you'll probably be more like 5-600 gph.