I think that right now, even with the poor acoustic piano already mentioned by Scott and others, the X1 is still a much better buy than the SD1.
Just my 2 Euros.

Andrea
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Korg Kronos 61 and PA3X-Pro76, Roland G-70, BK7-m and Integra 7, Casio PX-5S, Fender Stratocaster with Fralin pickups, Fender Stratocaster with Kinman pickups, vintage Gibson SG standard.