Titanium frames offer perhaps the best blend of all of the benefits of the other three frame materials. Its weight is comparable to aluminum tubing and is just as comfortable as steel or carbon while being nearly indestructible in terms of durability. So why isn't everyone riding titanium frames? In one word, it's price. They are expensive to make because of the process used to produce them.
In the past these frames have also not been the most appealing to they eye, which is a significant knock for most consumers when shelling out $8,000 dollars and up on a bike that might not be that flashy red you were looking for. Then again if you paint it red, how is anyone supposed to know you splurged for a titanium frame?
In the 1990s aluminum tubing was all the rage. But now with the invention of carbon fiber frames, aluminum has been put on the back burner. A large part of this has to do with the manufacturers—carbon costs more money to make but also sells for a higher price tag, which results in higher profit margins. This means more carbon bikes in bike shops.
Aluminum frames are actually some of the lightest and stiffest on the market, which makes them a great fit for those who like to race. In 2006, many of the professional cyclists on team CSC choose to race on the Cervelo Soloist aluminum frame as opposed to the carbon version. Bobby Julich even said it was one of the best frames he'd ever ridden. If it's good enough for the professionals, it should be good enough for you, not to mention that the price savings compared to carbon may win you points with your spouse.
The knock on aluminum is comfort, though the higher end tubing has become significantly more shock absorbing that its predecessors in the 90's. While the material can't be formed and molded as easily as carbon, it is still an excellent option for racers and time trialists looking for a stiff and responsive ride that won't rust or crack easily during a spill. If you're looking for aero everything, then aluminum probably isn't for you.
Carbon-fiber bike frames have a lot of good qualities going for them. For the most part they are pretty durable, offer a comfortable ride and are plenty light. The biggest advantage to carbon fiber though comes in the process of production. Because of the material, these frames can be molded and transformed to fit nearly any shape, which can be a huge benefit for someone looking for an aerodynamic triathlon bike or any of those Ferrari-like shapes offered by Colnago.