I think it depends a bit on what your doing with it. At 24" wide if it's an in wall tank with just one side visible then definitely tall. You can use reinforcements and glue to make a tall aquascape in a narrow tank. If your doing something like a wall divider or multiple side viewing the wider tank will definitely look more realistic than the tall tank and gives more room for big fish and wide corals.
If you are in a quandry and can't figure out what to do, there's a trick I've seen only a few people here use. Get some large cardboard boxes and cut them to size to represent the cabinet and canopy. Leave a gap where the tank will be. Take a look at it from all sides. This is the best way I know to help visualize what it is you are aiming for and what the results will be before you commit to anything.
Dave.M