Hi Andrew. The two cars are somewhat different in terms of size. The Mazda3 is larger, being a similar size to the Ford Focus. The Fiesta is the same size as the Mazda2.
All are good cars. Visit your local Ford and Mazda dealers to have a good look through each, and take them for a test drive if possible. You may well find you feel more comfortable in one than the other, or the package may be better on one than the other.