A wing is mega important. You would think it doesn't do much on 1/10th scale, but in fact it has more effect than on 1/1 scale. Air thickness doesn't scale, so in fact air to the 1/10th car feels a lot 'thicker' than to a 1/1 car. And on top of that, our buggy wings are huge compared to the overall size of the car, and it's mounted way back.
I always prefer a wide wing with less gurney over a narrow wing with a lot of gurney. The wide wing is stable, and the less gurney still allows it to turn.
Move the wing back and forward also has massive effect, in fast turns (for example turning in after the straight).
The more gurney, the more drag and downforce, which will make the rear end planted. Jumping will be more nose up. Another reason why I prefer a wider wing with less gurney.
Go here:
http://users.telenet.be/elvo/
Click aerodynamics and read the 10 pages :-)