I'm not talking specifically about triathlon frames, just frames which have aerodynamically shaped tubes. As I understand it, both types of frame are legal for road-racing under UCI rules.
So, why would one choose a traditional frame with round or "squoval" tubes over an aero frame? For example, why would one choose to ride a Cervélo R5 with (more) traditionally shaped tubes over a Cervélo S5 which would seem to be more aerodynamically sound?
Does it boil down to personal preference, or are there specific conditions in which one out-performs the other?