Carbon is much lighter than grp, to get strength from grp using csm you need quantity, the woven option is a bit tricky as poylester resin doesnt like to flex once built up and delamination issues can hinder you if not enough is used when creating voids and using cores. You will also find that grp has unreliable characteristics dependant on temp, look at the lines either side of the panel with the next one on say a cold day, then again on a hot one if you know what I mean.

Carbon is far superior, you can use different teqniques for gaining strength without using more material in the form of cores, ribs, basically when you introduct a void between 2 layers of infused fabric the result once cured is a very rigid part if done correctly. Also the epoxy is not affected by temp so once the part is made the size would stay the same.

It all depends on what you are after, if you are after cheap panels which are light but you wont be afraid of driving the car in anger in case of remortgageing to replace a panel then grp is defo the way to go, if you want to have the ultimate in strength and lightness then carbon/aramid is the way forward, whoever is doing these parts for you talk to them in detail as they will be able to adjust the part to your spec.

One last thing, if you are going to skin a grp bonnet with carbon, we have found epoxy has very poor bonding properties with polyester and delamination issues can occur so a carbon look vynal wrap may be a better soloution?

Hope this helps you