I have multiple good road bikes in each of steel, alu, titanium and carbon. I cannot tell the difference between them; the arm waving that folks do regarding 'ride quality' is solely due to tires, tire inflation pressure and the saddle. But the bikes definitely sound different.
Exception to my above assessment - carbon bikes are lighter. You can build a strong, stiff, and well-tuned road frame for the following weights:
- Steel: 1,500-1,700g
- Alu: 1,300g
- Titanium: ditto
- Carbon: <1,000g.
These weights are for roughly equivalent durability, strength and stiffness. These are not stupid light weights, but for bikes that can be ridden hard for years. Subtract 100-200 grams and you are close to the limit of the materials, for a somewhat more disposable bike.
All things being equal, carbon for the win.