Here it is as I see it. You are going to have to add water and do maintainance at regular intervals anyway. I only have a sump in one of my tanks and it is fresh water. I have no sw tanks yet, but I do know that regualr maintainance is a must with both. Now, it comes down to ease of maintainance. Is it easier to clean power filters or is it easier to clean sumps. In my case, I have power filters in my sump as well as pumps, media bio-balls, et al. The biggest advantage I can see to using a sump is the introduction of more dissolved oxygen to the system. For those doubters out there, my disolved oxygen in the "sump" tank is significantly higher than those using "normal", less exotic systems. In my case the "sump" tank can house more fishes.