I used to have an SD1. I played it for about a year. I sold it and got a Midjay and controller. I liked 'em all for various reasons. If you need 76 keys the SD1 is the way to go. I can't tell any difference in the sounds between SD1 and SD5. However the SD1 Plus may have some stuff I'm not familiar with.
Can't wait for the Audya.
DonM