As OhGizmo! explains, Duke Nukem Trilogy features three games, which have the same stories, but are presented in a different style on DS than on PSP. The PSP versions are M-rated, with content that will be "tasteful, but it'll still be Duke." The DS versions will be T-rated, and thus toned down a bit in terms of dialogue and gratuitous strippers.
The best part? The first game, Duke Nukem: Critical Mass, was actually there! There was a demonstration on both PSP and DS. Try that, Duke Nukem Forever. Try being playable on a game system.
Sony PlayStation Portable PSP-2000