I'd recommend right over the rear axle. The added weight will have the most traction impact with the least Cg pertubation in that position.

When you put the sand on the rear floor, you add some weight to both wheels - mostly to the rears, but some up front as well. Because you add it somewhat near the car's Cg, you feel the car handles pretty much the same as always. When you move the weight rearward, over the rear axle, you get more benefit in rear wheel traction, but you change the Cg a little bit, which changes the polar moment of inertia and the point the car will rotate about if it does spin.

For even better traction, you can put the bags at the very rear of the trunk. The lever arm formed by the trunk floorpan will give even better traction to the rear wheels, but you will significantly change the polar moment of inertia, making the car feel tail-happy when you do break traction.

When I add weight, I always do it over the rear axle - IMHO, its the best trade-off.