A quarantine will ive you a place to house sick fish in a stress free environment, and treat them with whatever means you choose (hyposalinity, copper, etc.) without effecting the chemistry of your display tank. It will also allow you to quarantine newly acquired specimen to avoid introducing any new disease into your aquarium.
A sump will increase the total water volume of your tank. It also gives you a place to hide filtration, heaters, skimmers, etc., cleaning up the look of your display tank.
A refugium is another possibility. It allows you to house your skimmer and heater, while having a compartment for denitrifying macroalgae such as chaeto. It will act as a form of filtration itself.
research all three possibilities and judge which would be best for your environment. Personally, I think a quarantine or refugium are your best bet.