Always buy based on lumens, watts is a number that is used to determine how much energy it takes to produce those lumens. It is important to pay attention to lumens per watt, or lm/w. The higher the number, the less energy you will need to produce the light. You will notice that some lights take less energy to produce the same amount of light, and this is something you should watch for. Typically, the higher the ratio, the better.