Although excellent products, mechanical filters such as Magnums and Emperors don't serve too much of a purpose in a salt water setup, unless you have a FO tank and are just looking to filter out larger debris from the water. In this same application, they also double as oversized powerheads for water movement.
They are also useful for clearing up water for newly setup tanks ("sandstorms"), or running media such as carbon or nitrate remover.
The most beneficial form of filtration for a salt water setup, be it reef, fish, or both, is a skimmer. Invest in a good one. It doesn't make any sense to spend your hard earned money on expensive fish and coral, only to have their health fail because of poor water quality.