There's very little difference with the SD1 plus over the SD1. The plus has a bigger piano sample if my memory serves me corectly and maybe a few other bits and bobs which are available to the SD1 if you upgrade to the latest software version.
I have a SD1 and have also heard the plus. To me there is little difference at all and it certainly isn't worth upgrading to. I didn't like the preset piano in the SD1 until I messed about with editing it.
It's about time a SD2 came out what with all the other manufacturers showing new products, I'm sure Ketron will have something up their sleave for later this year.