On airplanes, slats are definitely more effective. A slatted wing's stalling AOA will go from 15-16 degrees to mid 20s.
VGs on the leading edge, if placed at the right spot (just forward of the laminar separation point at high AOA), will increase the stall AOA about half that, to somewhere around 20 degrees.
Problem is, those things on the cars aren't really slats, which have to go at the leading edge. Those are more properly called guide vanes, which are not quite the same thing, aerodynamically.
You'd want to see a wind tunnel smoke test to demonstrate that something is happening, and they are not just sitting in the already turbulent air doing nothing. Or you could stick some tufts to the back window and observe them as you drive with and without the vanes.
You would never put guide vanes like that on a wing though, as the drag they produce would far outweigh any benefit, and they may produce nasty stall characteristics.