Chip's right. For oval racing, the 2.5 qt is plenty. These engines are really misers on fuel. I think that the smaller tank is more convenient to fit in an oval chassis, but if you were to run enduro races, obviously you'd need a bigger tank.
I'd agree unless you need the extra nose weight. I have the bigger tank and keep it full because the extra weight helps me. I use way less than a gallon of fuel for a full night of racing, maybe only a half gallon.