Utility
The Audi A3 is a better choice than the Mazda MAZDA3 as far as transporting cargo, especially if you frequently need to carry a lot of cargo or large pets.
Neither the Mazda MAZDA3 nor the Audi A3 has a marked advantage when it comes to seating capacity.
Dimensions
The Audi A3 is significantly heavier than the Mazda MAZDA3.
The Mazda MAZDA3 rides lower than the A3, which is a good thing for its center of gravity and ease of access.
The Audi A3 and the Mazda MAZDA3 compete for the same parking spaces.
Cost
The destination charge is a standard charge for transporting the vehicle from its point of origin to the dealer. The Mazda MAZDA3 is markedly cheaper to get to the dealership than the A3.
The A3 is tremendously more expensive than the Mazda with respect to MSRP.
Handling
The Audi and the Mazda MAZDA3 have approximately similar tires.
With their similar turning circles, the Mazda MAZDA3 and the Audi A3 very likely handle about the same into and out of tight spots.
Drivetrain
The Mazda MAZDA3's engine is quite a bit larger than the Audi A3's. Remember that more massive engines may use more gas than smaller ones.
The A3 packs quite a wallop compared to the Mazda MAZDA3 regarding torque, which is the force that lets you accelerate quickly.
The Mazda could learn a lesson or two from the Audi A3 in terms of pickup.
Comfort
Taller drivers will markedly enjoy the extra head room in the Mazda MAZDA3's front cabin compared to the Audi.
Overview
The Audi A3 and the MAZDA3 have the same basic warranty.
Other
The Mazda MAZDA3 produces fewer smog-forming pollutants per 15,000 miles than the Audi A3.