They're both great -- wishes has more "flowery" aerials, and illuminations more types of other effects. It kind of feels like Illum lasts a little longer, but some of that may be that I'm not too excited about the whole TV globe thing. But at Epcot you can have a beer while you watch, and at MK you have what can be a dreadful exit from the park experience after the show (depending on time of year and how tired you are, and whether you drive or take the bus). I like them both much more than Fantasmic, if that ever comes up. I'd probably give the edge to Illuminations, based on other factors than the fireworks (the MK fireworks have the edge). Try to see both, if there's any possible way (but I'm kind of a fireworks fanatic).