As far as I see, they are both based on file walker. (Storagenode section of this wiki: Debugging space usage discrepancies). Also see this comment: storj/storagenode/pieces/store.go at 52881d2db3732c07e3975237ff987e9f7c01bdf2 · storj/storj · GitHub (this is an estimation!).
If you are interested about the space usage, calculated by the satellite. I would recommend to query the sqlite database (see the mentioned wiki page).