I've owned both SD1 & SD5 and for me (my opinion only, so don't shoot me.)The SD5 is a newer sound engine, the sounds are newer, some of the the styles that are in the sd5, which came from the SD1 have been re-tweaked with the SD5 sound engine, they sound great, alive. The Sd1 is a great keyboard but its 8 years old. SD5 is 1 1/2 years old at best. SD5 has a combo of the X1, SD1, XD9 styles + many new styles also. I was a big ketron user for over 11 years and I have to say they are one of the best sounding keyboards that I've owned. Which everone you pick you will be happy.
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Ketron X1 (Oldie but Goodie)