I like them both, but it's really no comparison. The Prestige is damn near a masterpiece. The Illusionist is just a fun movie with a neat twist, & honestly the Prestige cast is so much better it's stupefying imho. I will give The Illusionist credit for being wonderfully & delightfully shot, & for what it is, a cloaked love story, it delivers quite well, but the Prestige is just relentless layers of well written & executed drama
I recall being rather disheartened that they both released around the same time, & seemed to be competing as a result. They're not even close to being a similar thing, despite the overall premise of European magicians at the turn of the 20th century-ish