Peat moss and ground peat are two entirely different things. Peat moss is just a bit removed from being alive, is fairly long strings of moss twisted up together. Ground peat is very ancient peat that has decomposed or composted into a nutrient rich material. The former can lower the pH of water, staining it in the process. The latter is a good additive in a thin layer under the substrate. The latter does provide some organic carbon when placed under most of the depth of the substrate, but the former doesn't do that, as far as I know. Of the two you mentioned, the sphagnum peat moss is the nearly alive stuff, and the brown earthy one is the ground peat stuff.