Hi mc: Perhaps I'm biased , but since you asked:

If (as in my case), having 76 keys or onboard sampling capabilities isn't a must, I recommend going with the Tyros over the SD1, as the Tyros styles are recorded at a much higher resolution (1,940 ppq), which contributes to making them sound more lifelike & alive (unquantized) than the styles in either the SD1 or Yamaha 9000pro.

In addition to concuring with Dreamer's observations, the other advantage (imho) of the Tyros over SD1 is it's very user friendly user navigation, OS interface, and Yamaha's excellent reliability record. I found the SD1, with its identically sized shaped buttons lined up in idental rows, a lot more difficult to navigate, especially in a live performance situation, and that Ketron's product reliability (especially the SD1) far below Yamaha's.

I auditioned both the SD1 and Tyros (and even the 9000pro) at GREAT lengths before finally deciding on the Tyros. After 9 months with the Tyros, I'm still thrilled with this keyboard. Need I say more. - Scott