I have spent excessive time with all of them.
Vocalizers:
Roland
Ketron
Korg
Yamaha

Styles:
Ketron
Roland
Yamaha
Korg

Lead (right-hand) sounds:
Yamaha - Ketron
Roland
Korg

Drums and Bass
Ketron
Roland

Op System
Yamaha
Roland
Ketron
Korg

Country Music
Ketron
Roland
Yamaha
Korg

Swing Big-band
Roland
Yamaha
Ketron
Korg

Jazz
Ketron
Roland
Yamaha
Korg

Midi files sound
Roland-Ketron
Korg
Yamaha

Portability:
Yamaha S900/3000
Korg PA800
Ketron
Roland

Value for dollar
Yamaha
Roland E-50, 60

It took minimum of one week steady hard work to learn enough about each of these to form opinions.

DonM