That entirely depends on what type of game you want because while they are both technically dungeon crawlers, they are most certainly not the same game mechanically.
Ravenloft: Cooperative game where all players work to defeat the boardgame and share responsibility of controlling the monsters during the game. Uses D&D's ubiquitous background. Mediumweight game (Meaning fair amount of rules intensity but not too deep).
Dungeonquest: Fairly quick game that Gimli would love to play. "Certainty of death, small chance of success... What are we waiting for?" You will likely want to avoid conflict with monsters since there is no reward for battling them.
FFG pasted on the theme of their own Terrinoth background onto the game (descent, runebound, runewars, etc). Somewhere between light and light mediumweight.
For more information, you should check out the following links:
Castle Ravenloft
Dungeonquest
(Disclaimer: Dont be too afraid of the BoardGameGeek interface. It is the most informative and useful source on boardgames you will ever find.)