Exhaust system modifications come in all shapes and varieties, each designed to produce a different result in terms of sound and power. Exhaust system engineering is a fairly complex science, but there are a few maxims that hold true regardless of the vehicle. At the end of the day, the choice usually comes down to a matter of cost and expectation.
A true dual exhaust is one that uses a separate exhaust tube running from each of the engine's two cylinder banks all the way to the end of the tube. Functionally, a true dual exhaust system is two systems running side by side. True duals are fairly rare, since most vehicles using them also utilize some sort of balance tube crossover to join the individual exhaust tubes near the engine. Turn-downs are just tips that slip onto the ends of the tubes and bend at a 90-degree angle toward the ground instead of pointing straight outward.
Cost almost always is a determining factor when looking at any sort of automotive upgrade. The investment required to install a true dual exhaust varies by application, but in many cases, it requires replacement of the entire exhaust system from the catalytic converters to the mufflers. Many cars and trucks also use a single catalytic converter, which means you'll need to install a second converter. The cost may run from a couple of hundred dollars for a custom-bent system from an exhaust shop to a few thousand. Turn-downs are far less expensive and install in minutes, and may even be free if your exhaust system is configured in such a way that you can simply cut the tubes to point them downward.
Horsepower and torque are a matter of airflow into and out of the engine, and your exhaust system can never do anything but cost you power past the ends of the header collectors or exhaust manifolds. You'll always make the most power by unbolting the entire exhaust system from the header collector or manifold and running full-loud. Dual exhausts exist explicitly for the purposes of enhancing flow and making power, so you're likely to see some improvement after installing one. Turn-downs just redirect exhaust gases and sound energy downward, which may cost you a bit of power by increasing backpressure at the exhaust's exit point.
Reducing backpressure in your exhaust system reduces air density and pressure in the system, which affects the speed of sound as it travels through the tube. The speed of sound drops as air density increases, so simply reducing backpressure in the tube allows sound waves to travel more quickly from the engine to the end of the tailpipe. This leads to a more distinct exhaust note; you'll hear the sound of individual cylinders firing instead of the indistinct buzz of sound waves overlapping. This also makes the exhaust louder, regardless of what muffler you use. Installing an H-pipe -- straight-tube -- crossover near the engine yields a distinct and muscular exhaust note, while an X-pipe crossover blends the sound waves for a more subtle tone.
Turn-downs can have a profound effect on your engine's exhaust note, mostly by using the ground itself and the space beneath your vehicle as an acoustic element in the system. When you turn the exhaust downward, you direct its energy at a sort of wall called "earth." When those sound waves hit the ground, the ground absorbs the higher-frequency and less-powerful waves, allowing the deeper and more powerful ones to bounce around under the vehicle. At this point, the area between your chassis and the ground turns into a resonance chamber like the inside of a drum. The farther your chassis is from the ground, the deeper and louder the sound of the exhaust, and parking on a soft surface such as sand or grass quiets and deepens the sound of the exhaust.
