Generally speaking, unless you are getting a wing for purely cosmetic reasons, don't bother. In fact, a wing is almost guaranteed to slow you down and kill your mileage. Why? Because that's exactly what they are designed to do.
And if you are getting it for cosmetic reasons, make sure you bolt it on properly, i.e. with Grade 8 bolts, through the decklid and backed by reinforcement plates. I have seen several wings tear off cars due to improper attachment.