The synth engine in the Xio has all the same programmable features as the X-Station with added extras like the X-gator. The Xio has fewer physical controls, however all the same features are available on the synth, using the various menus and pot banks.
The Xio has more routing flexibility than the X Station as an audio interface (eg, it is now possible to record a mic, guitar/line level signal and the stereo output of the synth directly to your computer via USB). Also, with the new Hybrid mode, you can play the synth with the keyboard, and simultaneously use all the assignable controls to control another hardware or software device in template mode.
The effects on the Xio can only be used on the output of the synth, and can not be used on the audio inputs as they can on the X Station, and the Xio keyboard does not have aftertouch. With the exception of these few limitations, the Xio can do everything the X-Station can and plenty more.