What do you want from your bike?
The Tarmac is designed to be a stiffer, more aero race bike.
The Roubaix is designed to be a more compliant riding bicycle for racing over rougher roads.
The Aethos is designed to be a lightweight bike for pure enjoyment.
This is kind of like comparing a sports coupe to an SUV to a sedan. Pick the one that fits your needs/purpose.