The material a bike is made out of is much less important than the design of the bike. What I mean is that a bike can be made out of any of the common materials to exhibit any quality or trait. As an example, consider aluminum. Many people will claim that aluminum bikes are harsh and overly-stiff, but it is the large tubes that gave them these traits, not the aluminum itself. On the flip side, the original aluminum bikes made by Vitus and others are often said to be the softest bikes every made. Again, it is the size of the tubes, this time very small tubes, that gave the bikes their riding characteristics, not the material.
Some very general comparisons. For bikes designed to have similar riding charcteristics, steel will be the heaviest, then titanium, then aluminum. Using the steel bike as a baseline, the titanium bike will have slightly larger diameter and thinner wall tubes, and the aluminum bike will have even larger and thinner tubes. For bikes that use identical size tubes (diameter and wall thickness), steel will be the stiffest and heaviest, then titanium, then aluminum.
Carbon fiber is a wild card because it can be manipulated much more than any metal and does not follow the trends above. It is true that carbon fiber can damp vibrations better than metal, but since the frame is isolated from the road by wheels and tires, and you are isolated from the frame by saddle, seatpost, shorts, stem, handlebars, tape, and gloves, it does not really make much difference.
Regardless of material, thicker wall tubes will better resist dents and other minor damage. An incident that seriously damages a bike will probably seriously damage any bike, regardless of material. A seriously damaged frame made of any material can be repaired, even carbon fiber, but it is generally not cost effecive to do so, even with steel.
In the end, I think it is most important to buy a quality bike, such as a Trek or Litespeed, that you enjoy riding. Don't worry too much about the material. Many people say fit is the most important, but remember you can fit on most bike sizes by adjusting the stem and seatpost. I think what they mean by "fit" is actually "handling," so test ride the bikes if possible and don't buy it if it doesn't feel right to you, even if the salesman says it fits you.
Sean
