Hi:

To my understanding, the Plus has an extra board that allows you to load STERO samples, and has a few more styles, and those, you can get them and load them in to the SD1, and as far it goes, the OS are a little different due to the internal map, EACH has their own OS.
I had 2 SD1s, my problem was the 64 note polyphony, and for my playing style was an issue, but other wise, I still like the sounds, styles and in fact, I did consulting for most Professional Ketron users in the Phoenix area.
If your playing style does not use a lot of notes and/or layer sounds, I would get the SD1 in a heartbeat.