The reason you switch the positive, in all cases is so you won't electricute yourself. Even in a house you don't switch a ground, or neutral.

Think about it. If you switch the ground and accidently lean on the positive, "YOU" become the ground. If your standing in a puddle of water, you could feel the effects of a fully charged battery. Although it is only 12vdc it could over excite ones pace maker.

Just make sure all of your circuits are fused correctly.

Don't skimp on price. Go down a get one that is going to cost more then 40 greenbacks (buy USA ) and you will have a switch that will last forever.