Love that one and it can be altered to fit the situation. Used it to great effect once by substituting the names of three friends :o)

Another one that is flexible, I first heard as "what's the difference between the Air Force and the Boy Scouts?"

...the Boy Scouts have adult leadership!

Substitute your favorite or not-so-favorite institution for "Air Force" and there ya go. Now, first one who posts "what's the difference between XMarks and the Boy Scouts. . ." will have to go sit in the corner!