Just to clear it up, it doesn't give you horsepower, it just adds more normal force to give you better traction... and unless you can already chirp your tires at 100mph it's not going to help you much. The real benefit to be gained is handling, but again that is minuscule really, unless you have a full kit and splitter and something like a WRC wing.
Just buy one that looks good. Wings West has a good urethane one (the kit on my car) or you could buy the Mach 1 lip if you want more of an OEM look. There's a few more like Kaminari and Steeda I think...