I have both... and I love both. There is really more in common between the 2 boards than differences. The voices and patterns are ~ 70 % common, with the SD5 having a slight edge in both. Most differences are minor, IMO, with the main difference being the number of keys, with the SD1 having 76 and the SD5 having 60.
One rap on Ketron boards is the OS, but that usually comes from those that have been weened on another manufacturer's KB. For me the Ketron OS is fine.
Try one or the other out... if you like what you hear, you'll be happy with either. Good luck.
Glenn