Cost
The destination charge is a standard charge for transporting the vehicle from its point of origin to the dealer. It costs almost the same to get the Civic to the dealership as the Mazda MAZDA6.
The Honda Civic is considerably less expensive than the Mazda MAZDA6 in terms of MSRP.
Utility
Neither the Mazda nor the Civic has a marked advantage in terms of seating capacity.
Comfort
The Mazda provides more head- and leg-room to your passengers than the Honda Civic does.
Dimensions
The Mazda MAZDA6 is tremendously more bulky than the Honda Civic.
Your wallet will hurt more every time you fill up the Mazda MAZDA6 than the Honda Civic because of its bigger tank.
The Civic and the Mazda MAZDA6 compete for the same parking spaces.
Handling
With their similar turning circles, the Honda Civic and the Mazda MAZDA6 most definitely handle about the same into and out of tight spots.
Drivetrain
The MAZDA6 has a more sizeable engine than the Honda Civic. Note that smaller engines typically get better gas mileage, but smaller engines can be stressed more.
Torque equals pickup, and the MAZDA6 will have picked up and gone by the time the Honda Civic finally gets moving.
The Mazda packs quite a wallop compared to the Honda Civic when it comes to torque, which is roughly equivalent to acceleration.
Warranty
The Honda Civic and the Mazda MAZDA6 have the same basic warranty.