For those who want a crisper keyboard, Aftertouch, and more controls and I/O, Novation offers the X-Station for just $50 more (list), but you give up the XioSynth's X-Gator, filter overdrive, and Hybrid mode. You won't be able to record the synth sound and external audio simultaneously. The same amount will buy the 49-key XioSynth, which could be a better option for playing live; I always miss a low E key when jamming with guitarists on a 2-octave keyboard. For more synthesis range in a compact keyboard, the Korg microX (which I reviewed in the January 2007 issue of EM) shines, but it costs more, lacks an audio interface, and falls far short of the XioSynth in MIDI control capabilities.
What do you get by choosing a Novation X-Station over the more affordable XioSynth? The most obvious benefit is more hardware controllers; the X-Station adds nine sliders, dedicated transport buttons, and quite a few more knobs, buttons, and LEDs. Its deeper case means the controls are spread out more clearly as well.
The X-Station's keyboard is higher quality and transmits Aftertouch. When I played the two synths side by side at NAMM, the difference was obvious (thanks to its semiweighted keys, though, the XioSynth still has a more solid keyboard than the typical 2-octave controller).
With the X-Station, you also get more jacks: an expression pedal input, MIDI In, an extra MIDI Out, S/PDIF out, and two combo XLR/1/4-inch inputs instead of the XioSynth's single XLR and 1/4-inch jacks. The X-Station can apply effects to incoming audio; the XioSynth cannot.