A sump gives you the option of adding liverock(wich will filter far more than any cannister)along with a spot to run carbon,put your heater,dose chemicals,do water changes,place a skimmer,setup a refugium...should I go on?..acclimate new arrivals,place unwanted critters until you can get them to the lFS,use a fan instead of a chiller,install a phosban reactor,place your thermometer,run filter floss...Im sure there are more. Ideally with saltwater we want to provide as much biological filtration as possible and remove ourselves and our animals from the money,time and risks that are all prevalent when we rely on mechanical filtration. In short nature provides the very best possible filtration for our tanks with very little cost incurred after setup. Get a sump with a chamber full of rock and a good skimmer and you are in business. Troll the buy/sell board here and other places and you will find a good sump cheap or you can make one yourself with a small tank or rubbermaid container,$300 is madness for a sump.