Sonic Cell has integrated USB audio interface and inputs, and two SRX slots. GW-8 has neither. GW-8 has double the ROM (expanded) but most of it is for the expansion 'world' area.

Sonic Cell has a voice editor and patch creator, GW-8 voice editing is much more basic. GW-8 style editing is basically non-existing.
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An arranger is just a tool. What matters is what you build with it..!