Unfortunately I'd need something a little more mobile.
I agree, because the light is more even from softboxes and much more controlled but Hairyduck is rightIf it were softboxes or umbrellas - I think I'd choose softboxes every time.
because, to obtain diffused specular highlights (which you can see through to the subject beneath them) you need a softbox or other diffused light source AT LEAST 3 times the size of the subject, and placed so close that it's almost out of shot. In fact, something with complex convex shapes like a car needs something much bigger than that,we used to use huge swimming pool sized softboxes
Unfortunately I'd need something a little more mobile.
For outdoor shots of cars, a large scrim (actually black material with thousands of tiny holes in it) is very effective and is the standard basic tool. In fact, except for 'splashy shots' of cars actually moving, I've never done outdoor shots any other way in sunshine.