Utility
Neither the Fusion nor the Mazda MAZDA6 has a marked advantage in terms of seating capacity.
Dimensions
The Ford Fusion's tank is roughly the same size as the Mazda MAZDA6's, though the Mazda MAZDA6's is a bit bigger.
The Ford Fusion and the Mazda MAZDA6 compete for the same parking spaces.
Convenience
You'll most certainly take the Mazda MAZDA6 for a fill-up roughly as often as the Fusion.
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 Mazda MAZDA6 to the dealership as the Fusion.
The Ford won't save you much money at the pump compared to the Mazda MAZDA6.
There honestly isn't much of a difference in gas mileage here, but if you must know, the Ford Fusion is a bit more economical than the Mazda MAZDA6.
With respect to MSRP, the Fusion costs almost as much as the Mazda MAZDA6 (though the Mazda MAZDA6 is a little cheaper).
Handling
With its tighter turning circle, the Mazda MAZDA6 is quite certainly sprier than the Ford Fusion, something to consider if you do a lot of city driving; however, remember that a vehicle with a tight turning circle may feel a bit twitchy on the highway.
Drivetrain
Torque is the force that lets you accelerate quickly, and in this respect, the Fusion isn't much more powerful than the Mazda MAZDA6.
Comfort
While the front cabin in the Mazda MAZDA6 offers a bit more head room than the Fusion, there honestly isn't much of a difference.
While the rear of the MAZDA6 offers a bit more head- and leg-room than the Ford Fusion, there frankly isn't much of a difference.
Overview
The MAZDA6 and the Ford Fusion have the same basic warranty.