Cost
The destination charge is a standard charge for transporting the vehicle from its point of origin to the dealer. The Toyota Avalon is substantially cheaper to get to the dealership than the Mercury.
The Grand Marquis requires a lot of gasoline compared to the Toyota.
When it comes to MSRP, the Avalon costs approximately as much as the Mercury Grand Marquis (though the Mercury Grand Marquis is a tad cheaper).
Utility
Drivers with frequent child-duty might favor the Mercury over the Toyota.
The Mercury Grand Marquis outpulls the Toyota Avalon by a sizeable margin.
Convenience
You'll definitely have to take the Mercury for a fill-up more often than the Toyota Avalon because of its tank size and gas mileage.
Comfort
While the front cabin in the Mercury Grand Marquis offers a bit more head room than the Toyota Avalon, there really isn't much of a difference.
The Toyota Avalon offers more head- and leg-room to your passengers than the Grand Marquis does.
Dimensions
The Mercury Grand Marquis is significantly more bulky than the Toyota Avalon.
The Toyota Avalon's tank is approximately the same size as the Mercury Grand Marquis's, though the Mercury Grand Marquis's is a little larger.
The Toyota Avalon and the Grand Marquis compete for the same parking spaces.
Handling
With their similar turning radii, the Toyota Avalon and the Grand Marquis most definitely handle about the same into and out of tight spots.
Drivetrain
The Toyota Avalon's engine is considerably smaller than the Mercury Grand Marquis's. Bear in mind that a smaller engine usually has to work a little harder; on the flip side, smaller engines typically get better gas mileage.
Torque is the force that lets you accelerate quickly, and in this respect, the Mercury isn't much more powerful than the Toyota Avalon.
The Toyota has no lessons to learn regarding torque from the Mercury Grand Marquis.
Warranty
The Toyota Avalon and the Mercury Grand Marquis have the same basic warranty.