There are many differences between the two, some symptoms overlap but they are not the same at all. Depression is mostly characterized by a severe low mood, negative thinking and intense emotions or the absence of emotions. Depersonalization is more of a sensation rather than thoughts or "feelings". You are detached from yourself and surroundings, a sensation you can truly feel, things look farther away or closer etc. Those experiences can result in depression or vice versa. In my opinion Depersonalization is the pinnacle of mental issues, when the mind cannot cope anymore, it's the absolute worst what exists it robs you from everything you are. True hell on earth.