You can really feel the weight difference between the two boards, although both feel like little armored cars. The FC660M = 700 grams, and the FC660C = 550 grams. For most, I think this is good, since the majority seem to be ordering the FC660C, and it will be a lighter burden to tote around (e.g. to and from work).
Also, the FC660M came in a plain, flat, tan box (unlike the photos), but included everything, including the extra 3 blank colored keys.