Well, they accomplish different things, so it depends on what you want to do. Carbon is a water "polisher" and a refugium is for nurturing pods and nutrient exporters like macroalgae. I would highly recommend a sump for a myriad of reasons, although I had a 20 gallon without a sump for a year or so and it worked out great. A sump gives you a place to hide your heater, protein skimmer, auto-top off, etc, etc, etc.