Cost
There really isn't much of a difference in gas mileage here, but if you must know, the Mazda MAZDASPEED3 is a little more economical than the RX-8.
The RX-8 is significantly more expensive than the Mazda MAZDASPEED3 in terms of MSRP.
Utility
Drivers with frequent child-duty might favor the MAZDASPEED3 over the Mazda RX-8.
Convenience
The Mazda RX-8's fuel tank will need to be replenished somewhat more often than the unit in the Mazda MAZDASPEED3.
Comfort
While the front cabin in the Mazda MAZDASPEED3 offers a bit more head room than the Mazda RX-8, there honestly isn't much of a difference.
The ampler interior dimensions of the Mazda MAZDASPEED3 make it considerably roomier for your passengers than the Mazda RX-8.
Dimensions
The Mazda MAZDASPEED3's tank is almost the same size as the Mazda RX-8's, though the Mazda RX-8's is a little larger.
The Mazda RX-8 and the MAZDASPEED3 compete for the same parking spaces.
Handling
With their comparable turning circles, the Mazda RX-8 and the Mazda MAZDASPEED3 most definitely handle approximately the same into and out of tight spots.
Drivetrain
The Mazda MAZDASPEED3's engine is tremendously more massive than the Mazda RX-8's. Remember that more sizeable engines may use more gasoline than smaller ones.
The RX-8 could learn a lesson or two from the Mazda MAZDASPEED3 with respect to torque.
Warranty
The MAZDASPEED3 and the Mazda RX-8 have the same basic warranty.