I think I can comment here, since I have a Midjay and had an SD1 in the past: to my ears the sound quality is identical and most samples are even the same.
In the past I had also an XD3 and agree with Flavie that is not in the same league, quality wise, maybe because they had to use shorter samples to fit in the smaller ROM memory (the piano multisample is the most striking example: the loops begin short after the attack and the decay is simulated via ADSR, but the end result is not very natural).
Hope this helps.