Personally, having read the novel and having seen both films, I prefer Dangerous Liaisons. Although the characters in the novel were closer to the ages portrayed in Valmont, I felt that Forman took too much liberty with the plot, and I detested Meg Tilly as Mme de Tourvel. Furthermore, I didn't think that Annette Bening's performance as Merteuil could touch Glenn Close, even though Close was much older than the original character in the novel.
To be fair, my former French professor, who was a specialist in 18th-century literature, had never seen a version of this story in film that she liked; she felt that the essence of the novel is its epistolary style, and that any film would be unable to recreate the exchange of letters.