An SC-55mkII rates higher on the coolness scale, IMO.
Not to go off on a tangent, but an interesting thing about the CM-300/500 is that they use a "GS-Hybrid" chip: the RLP-3037. According to schematics, this single chip integrates the MIDI sub-processor, PCM processor, and the RAM and ROM chips that are found separately on every other GS product (that I am aware of).
Why was the same chip not used elsewhere? Single point-of-failure? Production expense? Flawed design?