id go 4 TC. it looks better and its a chick magnet
Both have the exact same 0-60mph time of 7.4 seconds if they are both manual transmissions and if the Mazda 3 is using the 2.3L engine offered.
Both are reliable.
But the Mazda 3 is the best handling compact FWD car. It does handle much better and is more fun to drive then the Scion tC.
The Scion tC is owned by many younger drivers and the insurance is much more expensive. It is also 1mpg worse on gas and less practical.
The Mazda 3 is a better choice.
Maybe you can get a Mazda 6 with the same engine, but it will not be as fast because it is a bogger, heavier car.