Just wondering, the easiest way I can think of to give an example is this:
Need to figure out post count on a forum
Everytime someone posts their row updates a post_count field by 1, and then that number gets displayed.
You use count();(If I remember correctly) to tally all the posts that are started by that user and display that number.
Since most major forum systems use a post_count field in their tables I'm going to assume scenario A is more efficient, but does it really matter?