This is my first experience with Cubase and I'm totally blown away by how much more advanced (and complicated) it is compared to Reason. And how, for instance, to my sheer delight, the sounds are so much more usable and interesting.
However, taking into consideration what Colin says about it, they've made some changes and added a bunch of VST synths and stuff, but if you're already used to SX and already have a bunch of sound modules, it might not be necessary for you.