Kalimero,
I had an SD1 in the past and right now own a Midjay, along with a Tyros 2 and a Roland G-70, so I think I can comment on your question re. the SD1 sounds.
In general, the strong point of all Ketron instruments are the styles, that are particularly live, thanks especially to the second drum track that plays live grooves.
The sounds... well, let's say that they are "different", compared with Yamaha, Roland or Korg. They lack the same high fidelity quality but can still find their use in a musical context; on the Midjay the sounds seem a bit improved compared with the SD1 and my personal favorites are the organs, the woodwinds, the accordeons and a few others, like the vibraphone. Since you have asked about synth sounds, my opinion is that they are among the weakest ones of the bunch, so if you are looking for pads, synth brass, etc. my suggestion would be to look elsewhere.
All this might change with the release of the new Audya, next year, but of course nobody has tried one yet.
Hope this helps.