I have until recently mainly used the chase cam, but now for me it depends on the car/track combination and my mood.
I use whichever i feel the most comfortable with, which means I tend to use chase cam for faster/low slung racing cars and the roof cam for the slower/higher (and therefore can restrict my view) road cars (with GT4).
I think it also depends on my mood and the car; the roof cam is my default, especially if i wanna chase time (licences etc) or drive a FF car, but the faster a car is the more likely i am to use the chase cam.
If i have enough concentration, ill opt for the interior view where available, which i tend to use in combination with the chase cam, well thats what i do with GT Legends anyway.
Also it seems i'm one of the minority of people who prefer to have a steering wheel on screen when in cockpit view, even when im using a steering wheel (g25), i guess i concentrate on the screen not my hands (Again based solely on GT Legends).