The choice of buffer depends on the pH optima of the enzyme being measured and therefore the buffer that is closest in pH to the enzyme pH optima should be used. Usually cellulases of fungal origin with have a lower pH optima e.g. ~ 4 to 4.5 while cellulases of bacterial origin with have a higher pH optima e.g. ~ 6 to 7.5.
If the origin of the cellulase being measured is unknown then the recommendation is to to test the enzyme with both buffers and see which buffer generates the higher activity.