Carbon arms are not stronger. They are actually more rigid and weigh slightly less. Plastic arms flex more and therefore are less likely to break. Flex characteristics of suspension components have a lot to do with how the car is going to handle. Slightly more flex is going to generate more traction, so the standard plastic components work better on low to medium grip tracks (what the majority of people race on) and therefore that is what most kits come with. The carbon pieces are more rigid, and help take some of the twitchyness out of the car on high grip surfaces