Have owned steel, aluminum, ti and now have ordered a custom carbon single.
Steel is real, heat treated alu was a bit harsh riding but much stiffer, ti was nice and stiff, and not as harsh.
After having ridden a carbon fiber tandem for about 5,000 miles, I am sold on stiffness/weight/comfort of carbon. Hence the c/f single on order.
Isn't it nice to have choices?