refugium is far better than a wet/dry. not only will the bio balls in the wet dry get caked up with calcium over time, and you will either want to shoot yourself in the face cleaning them, or buy new ones, but they don't have the same kind of room that you would get with a refugium, leading to less filtration. You don't have the macro algae or any other plants you decide to put in there. You don't get the pods reproducing and feeding your tank(the drip tray in a wet dry will actually kill any of them off that make it into your overflow), you have far more room for rock, plus your sand bed in a refugium, where a wet dry, you could put rock, but they are typically always smaller. Wet/dry's are great for freshwater, but when it comes to saltwater, you can't beat the refugium. A wet/dry would still work, but not nearly as effectively