C7 has +-50u/300mm,
C5 at 300mm has +-25u with 20u fluctuation.

Accuracy grades C0 to C5 are defined in the linearity and the directional property, and C7 to C10 in the travel distance error in relation to 300 mm.
(in accordance with the JIS standard JIS B 1192 (ISO 3408))

There are much more important things on machine than C5/C7 selection.