Carbon vs steel: Carbon is lighter, can be made more aero. *Possibly* better ride, by making the frame torsionally stiff, but softer in longitudinal flex. We know that small-tube steel can be made to ride well, but it can be more noodly in hard climbing and sprints, which matters in racing. My aluminum Cannondale 3.0 crit was notably stiff in torsion (I think best ever until that point) but also rode hard, but that may also have been due to the steep angles and short wheelbase. Some have said newer aluminum 'Dales ride well, but the aluminum has gotten very thin, and decades-durability can be an issue. I think steel has had a resurgence due to to that. Titanium was supposed to be the perfect compromise, midpoint in density between aluminum and steel, but high fatigue strength, and the relative price of Ti has come down. So from my view:
- I stay away from carbon due to its known fragility in frame hits. But I don't race.
- Not planning to get another aluminum road bike. I have a couple aluminum frame 700c hybrids suitable for touring or townies, which is a good application.
- I like my 4130 steel 20" folder.
- I wouldn't mind a titanium frame if well built and good price. However, if touring in remote areas, heck, even near a city, titanium welding repair might be difficult to find, versus steel or aluminum. Another mark against carbon.