Hi Chris,
Acids are corrosive to ferrous metals but generally kinder to aluminium alloys than are alkalies. Since these alloys are more commonly found in CC machines the acidic rinse score well in most cases.
The strength of the rinse will also have a bearing as will whether the machine is flushed after use.
One positive advantage of acids is their de-scaling effect, so though I normally use only plain water, If I have not used an acid rinse for technical reasons, I make a point of putting some through the system occasionally to combat lime-scale build-up.