I don't own neither of the two but I've been hearing a lot of complaints about Mazda 3 1.6V's fuel efficiency.... they said its disappointing for a 1.6L engine

on the other hand, I've heard a lot of good feedbacks regarding Vios 1.5's fuel efficiency, even my officemate who has a Vios 1.5 is very happy this cars fuel economy.

Hands down Mazda 3 is the much better looking than the Vios, but it will all come down to what you really want.