For most people it's purely aesthetics, if you come from a Mountain Bike background probably the sloping top tube looks fine. If you're from a road-cycling background then the flat TT may look better. If you want to have higher handle bars then there are definite advantages to the sloping TT.

From a design point of view it is easier to accommodate a longer head tube if the top tube is sloping, this allows for use of a longer steerer tube on the fork and higher handlebars. If you raise the TT on a classic frame it quickly turns the TT in to a high-jump bar for many of us. Otherwise with a classic frame you can always change the fork for one with an un-cut steerer tube and add a lot of spacers, but that really looks bad.

Try playing around a bit with bike design and see what the effect of different top tubes is, etc. using BikeCAD (maybe you'll end up getting a custom frame built to your own design!).

See: http://www.bikeforest.com/CAD/bcad.php