Hi Diki, There are lots of the synth sounds (leads, pads and basses) in the Gw8 which I really like but are not in the BK9 or G70.
Also - I like to use the GW8 over the BK9 and G70 sometimes because of its portability. I have it connected to a laptop and can take it on trips easily / move it around the house etc
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Roland G70 / Roland BK9 / Roland GW8-L / Yamaha Motif XS / Technics KN6500