Body kits serve the dual purpose of enhancing the looks and improving the aerodynamics. For a street car the primary purpose tends to be the looks. Performance is secondary. Therefore the main consideration while choosing body kits are the looks and style, besides other considerations such as durability and cost.
However for a racing car performance cannot be compromised. And the racing car body kits should serve to improve the aerodynamics. One of the first considerations is that the body kit should be light weight. A heavy body kit will drag down the performance of the car.
For the aerodynamics enhancement it is important to consider body kits that have been tested to improve the aerodynamics. It should have smooth contours that allow the air to bend and pass through easily without creating a large drag force. However it is difficult to judge how aerodynamically efficient a body kit is just by looking at it.