I went to TQRC's site, and see only the D3, Zero and XI Sport.

If there is any difference, it would be the sport has an FRP chassis, rather than a plastic or carbon chassis. Fiber-Reinforced Plastic chassis are more durable, and has a bit more flex, than a carbon chassis and are slightly more durable than a plastic chassis. FRP chassis are also not electrical conductants, so you won't get "glitching" with them, as some servos do, when mounted on carbon chassis.

I have a couple of chassis made from FRP, and have not had any problems with them. They aren't as nice as carbon chassis, but they work well, and are at least a step-up from plastic chassis.