They are both a library with solved solutions. Main difference being that GTOW focuses on huge amount of spots solved and you can look up whatever runout you want, while DTO focuses on high accuracy in less spots which are only avalaible through trainer, so you cannot decide to look how for example K36TJ plays.
DTO Poker is free to download and play, with two premium subscription levels for more advanced players: The 'Grinder' level focuses on teaching a GTO-based approach across various situations.