The radius of the wheel would make the mousetrap car much faster if it was bigger, it would be able to cover a larger distance without longer string or leverage. The friction between the wheel and the surface. This is important as we want to have speed and not be slowed down by friction. Stability is important as if the vehicles move left and right and would cause the vehicle to move unnecessarily and if it topples it would be useless. Leverage is very important it would affect the distance and speed of the car as it will cause more tension on the string causing more power to the wheel. Having easy to get materials is useful as finding materials are troublesome and time-consuming for the project causing less time spent on construction.
The radius of the wheel and the friction between the wheel and ground are the top priority as they will affect the speed and distance travelled by the car. The leverage is less important by a bit as it will help with the speed and distance but not by as much as the radius and friction between the wheel and ground. The stability is not as important as it does not really affect the result by much. Easy to get materials is the least important as the difficulty of getting materials is the least of our concern.
The reason why we pick Afiq's design as the leverage is fairly long so that there will be more tension to pull the wheel to a good distance and he uses the 2 similar CD to balance the back wheel so that it would not have too much weight held on one side and lose a lot of the energy. And using the lego he can reduce the energy lost through friction and lego is lightweight so is will have lesser friction and lesser friction mean lesser energy loss. As for lego even if it is expensive and not as easy to find, it is not so important as it does not affect the result at all.