It's a shame that these two movies came out so close to each other, because they're both very good, but they're really very different movies and so it'd be sad if someone had not seen one or the other because they'd already seen a movie titled The Something about late 19th century magicians.
The Illusionist, in particular, felt much more like a fairy tale to me -- a story of a prince and a baroness and a poor young man who loved her. Paul Giamatti's police inspector grounds the story in that turn-of-the-century tension between logic and spirituality, but the essence of the story could have been set in any time when love fought against power.
The Prestige, on the other hand, seemed so grounded in a specific time and place that it (SEMI-SPOILER!) really pissed me off when a major plot point was revealed to be pure science-fiction. I did, however, delight in the torturous story-telling of the journal-within-a-journal, and the final reveal was extremely surprising and satisfying.
FuzzyCo grades: A and B, respectively.