There are a few different reasons to have a refugium.  You may want to have additional water capacity, for this it is just a small amount more.  If you want it for growing pods and for nutrient export (the macro algae in the refugium outcompeting with the microalgae [the bad stuff] for nitrients) it will work out fine.  It really isn't designed to remove particulate matter but you can add an area to have a filter pad or sponge.  If you go with the larger model you may also be able to place your heater, thermometer, and a bag of carbon in it as well.  If you add either a pad or sponge though keep in mind that it will need to be cleaned a minimum of weekly in order to keep it from becoming a nitrate factory.  HTH,  Skip