Just personal preference.

I agree with all that has been said about cotton. Here in New Orleans we know a thing or two about humidity and perspiration. If I ride in summer with a cotton t-shirt it quickly becomes completely soaked and literally dripping. With the extremely high humidity we have we don't get ANY evaporative cooling effect because the air is so saturated nothing evaporates! The saturated, dripping cotton t-shirt actually becomes a barrier, holding heat in rather than letting it escape. If I ride with a t-shirt of Coolmax, Hydroduct or other wicking fabric under my jersey I arrive with the t-shirt still be very wet, but not so saturated it is dripping. Many times it is just damp where a cotton t-shirt would be saturated. I am a BIG believer in wicking fabrics. Down here it is almost a matter of survival in summer. I have visited drier areas where this is not such a big deal. Where it is really dry and evaporation is rapid cotton may be fine.