It depends who you talk to, but lots of people claim that the Road BBs are smoother, while the Mountain BBs are more geared towards mud and whatnot.
... Let's face it, they all use the same dang bearing covers.
Now, mind you, there are some very heavy duty Mountain BBs with Quad Bearings and fancy things like that... but that is out of the scope of this discussion.
One thing to keep in mind, however, is that Mountain BBs from some manufacturers nowadays are a single '68/73mm' unit... They use a shim on the drive side to fit in the smaller size. If you guessed that this totally messes with your chainline, you get a gold star.
Course, Two Piece Extrenal Bearing setups for mountain bikes have spacers on both sides, and you can mess with the chainline at your leisure...