CarloM, what saddle did you end up going with?

TREK/Bontrager has a "biodynamic" saddle posture categorization system which ranges from Posture 5 Leisure to Posture 1 Aerodynamic.

Posture 5 - Leisure
Posture 4 - Fitness
Posture 3 - Performance
Posture 2 - Aggressive
Posture 1 - Aerodynamic

The Bontrager Montrose line, including the Montrose Comp saddle that comes standard on the FX Sport 5/6 is considered to be part of Posture 3 Performance. The Sport and Commuter Comp saddles are part of Posture 4 Fitness.

Note that the FX Sport 4 has the same more aggressive fitness geometry of the 5/6 yet it comes standard with the H1 saddle which to my knowledge is a Posture 4 Fitness saddle. You might indeed be more comfortable on the Sport or Commuter Comp.