It's hard to tell at this point how the QX30 will be different from the Q30, which we have seen in concept form. I have read that the QX30 will compete directly with the Audi Q3. Perhaps a good way to understand the difference between the Q30 and QX30 is to look at the difference between the Audi Q3 and the A3.
Audi A3
Audi Q3
Infiniti Q30 Concept
Audi A3
Audi Q3

Infiniti Q30 Concept
