Hercule Poirot was the main character in Agatha Christie's first novel (written in 1916, published in 1920). She wrote a total of 33 novels and 55 short stories starring Poirot. His first movie was in 1931, and had his first series on "Mystery" in 1989-90.
Miss Marple was created around 1930 and had 13 novels and 12 short stories. She wasn't seen in a movie until 1961, but was seen on PBS as early as the 1985-86 season.
For trivia lovers: David Suchet has portrayed Poirot in more than 60 PBS films although others have played the role on TV and in movies.