If you are targeting an aero road bike, then any of the major brands are functionally equivalent. Each has its own brand spin on the frame design, but at that level, they are super close. Then, the only other major differences are component mix.
Most bikes in the low $3K range will have 105 or Ultegra mechanical. The S3 is $4K and Ultegra Di2. Is Di2 a priority for you? Also, have you thought about disc brakes versus rim brakes?
These are the quick pick aero road bikes...
- Trek Madone
- Specialized Venge
- Cannondale SystemSix
- Giant Propel
- Argon 18 Nitrogen
- Canyon Aeroad
- Scott Foil
- BMC Timemachine 01