You can have both. They effectively do the same thing when the skimmer is run very wet. The point of the skimmer is to concentrate the proteins in the water, and remove just them, not so much the water, so you don't have to do waterchanges as frequently. There are additional good things about skimmers too besides just concentrating and removing waste, like the aeration of the tank water (raises pH, helps ensure everything in the tank can breathe easily, etc).