If you get a clear look, you can notice that the platform used has no bumpy forms, in other words, the plastic has no messes here and there, it's a simple, completely flat surface and the motor is outside the car. (Dirt/little debris) might get caught in the mosaics of the body in the 4x4 body. Though probably that those mosaics of the body add more rigidity to the body.
Now look at the battery placement, in the 2WD it is perfectly in the middle of the car, so the weight is properly balanced in my opinion, now the 4x4 has the battery compartment at one side of the body, weight issues? (depending on the weight of the battery used, lets say a NiMh 8000 mAh that weights much more than the stock traxxas battery for instance.)
All this is in my opinion, thoughts I wanted to share.