Hard decision! If I could only see one and I had never seen either, I would have to go with .... Fantasmic!
I think it is because it is pretty unique. It has a more of a story that is followed, the spray of water with film clips is just pure Disney-lover amazing. If you love the characters, you have a great chance to see little-seen ones. Mickey is there! You can do the dinner package, and that gives you a better chance for a seat. You are seated the entire time and there are some entertainers wandering around while you wait. Sometimes when you get a great spot at Epcot for Illuminations, a rude person or family will come up and stand very close, shoving in as you stand.
That being said, we love Illuminations, and I would watch it if at all possible.