Very complex question Chad. Many metal dome tweeters are better suited for off-axis (outside or near the limits of a given domes' horizontal and vertical dispersion). Titanium is harder and stronger than aluminum by weight. This is strictly pure titanium and aluminum now... there are aluminum alloys out there that are as strong or stronger than titanium by weight. But then there are also titanium alloys as well. To simply state that the material used in a dome or compression driver is the end-all determining factor in it's sound and response is misleading at best. There are many factors that go in to making a high frequency driver. But on a general scale, titanium will be crisper and more detailed (some might say shrill) than aluminum due to it's superior stiffness-to-weight ratio and will also (generally) handle more power and will be more immune to cone breakup. This is why titanium is very common in pro sound high frequency compression drivers. Aluminum "warms" the sound a bit to most people, and is seen as a decent halfway point between the smoothness (but decreased very high frequency response) of a silk dome. Buying a very high quality dome is definitely more of a factor than the material used in the diaphragm. There are high end examples of domes made with each of these materials, as well as beryllium, magnesium and polyester. It all comes down to the build quality, intended use, output requirements -- and your ears.
