Whereas the colon and dash imply a continuation
of the same idea, the semicolon makes
better readers expect a contrast:
He went swimming;
she did the dishes.
The Knicks play
zone defense; the Pacers prefer one-on-one.
The semicolon also invites readers to consider the reasons or circumstances
for the differences. In the first example, good readers will recognize
not just two statements of opposing facts, but also an implication that
(as usual?) the man is having fun while the woman is stuck doing the work.
The second example invites the reader to consider the differences in players,
coaches, and/or strategy which would account for the different preferences.
Depending on his or her purpose, the writer may or may not go on to explain
the differences. In some cases, for example, part of the fun of reading
and writing is that the experienced reader catches with implications of
the statement without being specifically told. In other cases, the sentence
acts as a topic sentence for a discussion of the differences