I am so glad this question was asked.

Just to let you know, I have owned a PA-80, VA-7, EM-2000, G-800, and now have a Tyros and SD-1

I would have to say if it was possible to morph the 2 keyboards, and a couple of additions you would have the arranger board of the decade.

Tyros wins in the
Operations
screen
layout
weight
half the styles
half the sounds
Polyphony
quick layer ability
front mic volume

SD-1 wins in the
vocalizer
xlr input
dynamic range
extra keys
more compact design
half the styles
half the sounds
hard drive interface with certain functions

SD-1 bad points
layout, still not easy enough for live play
polyphony
operations
screen
info on screen