You´re talking about the r34? I also had to make my mind about those two. The v-spec´s racing mod was more pleasent to the eye and since the weight increase wasn´t much I kept it. I have no idea if the weight really makes a difference.
This goes for all Skylines ie R32,33,34
it's already a heavy car at about 3400lbs stock and the difference between the V spec model is around 50 lbs or so. the V spec model doesn't seem to be much different than the regualars. the thing is it's adding more weight to a already heavy car but at the same time the 50 or so lbs is insignificant if compared to the weight of the car...
They measure the weight outside the game, and a limited-slip system would add fifty pounds. Since it's the V-specII, it might have other, interior stuff as well.