From the experience of chisiang, a retailer that knows bodykits:

In the market, bodykits are made from 4 main type of materials (of course there's the carbon fiber option) namely: ABS Plastic, PU, FRP, Fiber Glass.

The best bodykit materials have to be either ABS Plastic or PU (Polyurethane). The finishing from these are simply flawless. Fitment is also perfect. However, the downside is price. The molding cost is very high. In addition, if they are damaged or knocked, repair is almost impossible. PU is also very heavy. Only OEM manufacturers like Mugen, have the money and volume to produce these bodykits.

At the other end of the spectrum is the Fiber Glass bodykit. Molding cost is extremely low, material is cheap, and fabrication is fast. This is the choice replica makers. However, finishing is also the worst. It also suffers alot from tearing and breaking.

In the last 5 years or so, Japanese bodykit makers started mass adoption for FRP (fiber reinforced polymer) for their bodykit. These bodykits are commonly mistakened for fiber glass but they are not. FRP inherits part of the smooth finishing of the PU and the regidity and light weight properties of the fiber glass but molding cost is still relatively high. Repair is also possible.

When choosing a bodykit, PU and ABS are no doubts the best but when cost is concerned, they are luxury. And since repair is almost impossible, mom and wife should be kept out at all cost.

Purely fiber glass bodykit is cheapest to own and they can be fixed easily if broken. However, if you do not have a good paint shop in your area, your car may ended up on laughatrice.com due to the poor finishing.

FRP offers a middle ground. Without losing too much on finishing, and at a slightly lower cost from PU and ABS, you can have the killer look and yet the option to repair if damaged.

A new kind of hybrid rubbery and flexible material is beginning to surface. You can bend it and step on it and it will bounce back into shape without tearing! After a few sales of those kit, we realised that these bodykit will need a special kind of flexible paint. And also, under hot summer weather, they lose their form easily.

Here's a good site where they list down the various type of bodykit materials' properties in table form.

http://www.sportcompactonly.com/body-kits-guide.aspx