I have no experience with SD1 or X1 whatsoever and I TRIED but no dealer carries this thing in my state: George Kay is too far.
Sk880 or WK8 is an excellent keyboard, however, it is an older keyboard that I would not consider right away.
1) some are the sounds are decent but now in comparison to SD1 or some of the newer Yamaha, they are lacking.
2) GEM are always better than everybody when it comes to sequencers.
3) GEM lacks ballad styles
I listened to some demo of X1 and I liked it. If you really want a keyboard NOW, and you want to select between GEM and SD1, I would pick SD1 right away.