Chronologically speaking, Rachmaninov spent around half his life in the 20th Century. Though, his pieces are composed and meant to be played in a late Romantic style. If you were playing Rachmaninov for say, an audition where pieces from different periods are required, you may want to check if they consider Rachmaninov 20th C or Romantic.
Debussy was an impressionistic composer who is considered by some to be late Romantic also.
With regards to the piece you have mentioned, I could say they could both be considered in the Romantic era.