i have both and i would like to give a fair comparison
Cabin Space : Vitz
Drive Comfort : Vitz (i wish joy was automatic)
A/C : Joy
Mileage : Vitz ( Till now i am not getting that good of an average on my joy and Nexus people told me that by first check up average would increase right now its freaking 8km/l with A/C 100%)
Ride Comfort : Both are equal (notice i said drive comfort and ride comfort, ride comfort is based on experience of pessengers in the car, how they fit in and how is their experience from point A to point B)
Overall : Vitz is much better vehicle than Joy, although joy has its own merits when you compare it with other options available to us