Peeping Tom reminds us of the scary, primitive power of movies by putting us, as viewers, in the position of its peeping protagonist. As the psychopath sets up his victims for the kill, we see them through his eyes.
While Hitchcock's masterpiece is unquestionably the more mainstream and formally groundbreaking of the two films, it's arguable that Peeping Tom is the more thematically profound and insightful work. Just don't expect to feel good after watching it.
If anything deserves the "dark masterpiece" tag, this does: a brilliant satirical insight into the neurotic, pornographic element in the act of filming, more relevant than ever in the age of reality television and CCTV.