I believe its very simple to decide.
Prefer to buy which you like most. Don’t let others to take your decisions.
If you see Vitz on road and your heart start pounding bit higher and it brings smile on your face, then go for Vitz.
And if same situation happens for Mira, then go for mira.
Generally people who buy 660cc cars, initially they enjoy the good fuel economy. But sooner or later they move to bigger engines like 1000cc at-least. Because smaller cars lack lots of power too.