Personally, I think the answer to your question depends on a couple of things:
Are you a big fan of Disney's animated movies? Have you seen most of them, including newer movies like Pochahontas? If so, you're likely to enjoy Fantasmic more than someone who can't place what they're seeing into context.
What interests you more, story or spectacle? If you tend to watch movies with lots of special effects, go for Illuminations.
Personally, though I know there are plenty of exceptions, I think more children would probably prefer Fantasmic, while more adults would prefer Illuminations.