NYC Maggie Backstedt fan
Join Date: Sep 2006
Location: New York City
Bikes: Trek road and hybrid bikes
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The mountain bike post is longer.
If the diameter is the same, you can use either without a problem.
Not a universal rule, but if you "show" a lot of post, so that it is close to the max line, ride a particularly large or a compact frame, some folks recommend you use the "mountain" post, so that the bottom of the seatpost when installed is fully south of the bottom of the intersection of the seat tube and top tube.
Of course, the mountain post weighs more.